Record realtime close marker on replacement (#13058)

## Summary
- record a realtime close developer message when a new realtime session
replaces an active one
- assert the replacement marker through the mocked responses request
path

---------

Co-authored-by: Codex <noreply@openai.com>
Co-authored-by: Charles Cunningham <ccunningham@openai.com>

codex-rs/core/tests/suite/snapshots/all__suite__compact_remote__remote_compact_resume_restates_realtime_end_shapes.snap

@@ -0,0 +1,24 @@
+---
+source: core/tests/suite/compact_remote.rs
+expression: "format_labeled_requests_snapshot(\"After remote manual /compact and resume, the first resumed turn rebuilds history from the compaction item and restates realtime-end instructions from reconstructed previous-turn settings.\",\n&[(\"Remote Compaction Request\", &compact_request),\n(\"Remote Post-Resume History Layout\", after_resume_request),])"
+---
+Scenario: After remote manual /compact and resume, the first resumed turn rebuilds history from the compaction item and restates realtime-end instructions from reconstructed previous-turn settings.
+
+## Remote Compaction Request
+00:message/developer:<PERMISSIONS_INSTRUCTIONS>
+01:message/user[2]:
+    [01] <AGENTS_MD>
+    [02] <ENVIRONMENT_CONTEXT:cwd=<CWD>>
+02:message/developer:<realtime_conversation>\nRealtime conversation started.\n\nYou a...
+03:message/user:USER_ONE
+04:message/assistant:REMOTE_FIRST_REPLY
+
+## Remote Post-Resume History Layout
+00:compaction:encrypted=true
+01:message/developer[2]:
+    [01] <PERMISSIONS_INSTRUCTIONS>
+    [02] <realtime_conversation>\nRealtime conversation ended.\n\nYou are...
+02:message/user[2]:
+    [01] <AGENTS_MD>
+    [02] <ENVIRONMENT_CONTEXT:cwd=<CWD>>
+03:message/user:USER_TWO

codex-rs/core/tests/suite/snapshots/all__suite__compact_remote__remote_manual_compact_restates_realtime_start_shapes.snap

@@ -0,0 +1,24 @@
+---
+source: core/tests/suite/compact_remote.rs
+expression: "format_labeled_requests_snapshot(\"Remote manual /compact while realtime remains active: the next regular turn restates realtime-start instructions after compaction clears the baseline.\",\n&[(\"Remote Compaction Request\", &compact_request),\n(\"Remote Post-Compaction History Layout\", post_compact_request),])"
+---
+Scenario: Remote manual /compact while realtime remains active: the next regular turn restates realtime-start instructions after compaction clears the baseline.
+
+## Remote Compaction Request
+00:message/developer:<PERMISSIONS_INSTRUCTIONS>
+01:message/user[2]:
+    [01] <AGENTS_MD>
+    [02] <ENVIRONMENT_CONTEXT:cwd=<CWD>>
+02:message/developer:<realtime_conversation>\nRealtime conversation started.\n\nYou a...
+03:message/user:USER_ONE
+04:message/assistant:REMOTE_FIRST_REPLY
+
+## Remote Post-Compaction History Layout
+00:compaction:encrypted=true
+01:message/developer[2]:
+    [01] <PERMISSIONS_INSTRUCTIONS>
+    [02] <realtime_conversation>\nRealtime conversation started.\n\nYou a...
+02:message/user[2]:
+    [01] <AGENTS_MD>
+    [02] <ENVIRONMENT_CONTEXT:cwd=<CWD>>
+03:message/user:USER_TWO

codex-rs/core/tests/suite/snapshots/all__suite__compact_remote__remote_mid_turn_compaction_does_not_restate_realtime_end_shapes.snap

@@ -0,0 +1,36 @@
+---
+source: core/tests/suite/compact_remote.rs
+expression: "format_labeled_requests_snapshot(\"Remote mid-turn continuation compaction after realtime was closed before the turn: the initial second-turn request emits realtime-end instructions, but the continuation request does not restate them after compaction because the current turn already established the inactive baseline.\",\n&[(\"Second Turn Initial Request\", second_turn_request),\n(\"Remote Compaction Request\", &compact_request),\n(\"Remote Post-Compaction History Layout\", post_compact_request),])"
+---
+Scenario: Remote mid-turn continuation compaction after realtime was closed before the turn: the initial second-turn request emits realtime-end instructions, but the continuation request does not restate them after compaction because the current turn already established the inactive baseline.
+
+## Second Turn Initial Request
+00:message/developer:<PERMISSIONS_INSTRUCTIONS>
+01:message/user[2]:
+    [01] <AGENTS_MD>
+    [02] <ENVIRONMENT_CONTEXT:cwd=<CWD>>
+02:message/developer:<realtime_conversation>\nRealtime conversation started.\n\nYou a...
+03:message/user:SETUP_USER
+04:message/assistant:REMOTE_SETUP_REPLY
+05:message/developer:<realtime_conversation>\nRealtime conversation ended.\n\nYou are...
+06:message/user:USER_TWO
+
+## Remote Compaction Request
+00:message/developer:<PERMISSIONS_INSTRUCTIONS>
+01:message/user[2]:
+    [01] <AGENTS_MD>
+    [02] <ENVIRONMENT_CONTEXT:cwd=<CWD>>
+02:message/developer:<realtime_conversation>\nRealtime conversation started.\n\nYou a...
+03:message/user:SETUP_USER
+04:message/assistant:REMOTE_SETUP_REPLY
+05:message/developer:<realtime_conversation>\nRealtime conversation ended.\n\nYou are...
+06:message/user:USER_TWO
+07:function_call/test_tool
+08:function_call_output:unsupported call: test_tool
+
+## Remote Post-Compaction History Layout
+00:message/developer:<PERMISSIONS_INSTRUCTIONS>
+01:message/user[2]:
+    [01] <AGENTS_MD>
+    [02] <ENVIRONMENT_CONTEXT:cwd=<CWD>>
+02:compaction:encrypted=true

codex-rs/core/tests/suite/snapshots/all__suite__compact_remote__remote_pre_turn_compaction_restates_realtime_end_shapes.snap

@@ -0,0 +1,24 @@
+---
+source: core/tests/suite/compact_remote.rs
+expression: "format_labeled_requests_snapshot(\"Remote pre-turn auto-compaction after realtime was closed between turns: the follow-up request emits realtime-end instructions from previous-turn settings even though compaction cleared the reference baseline.\",\n&[(\"Remote Compaction Request\", &compact_request),\n(\"Remote Post-Compaction History Layout\", post_compact_request),])"
+---
+Scenario: Remote pre-turn auto-compaction after realtime was closed between turns: the follow-up request emits realtime-end instructions from previous-turn settings even though compaction cleared the reference baseline.
+
+## Remote Compaction Request
+00:message/developer:<PERMISSIONS_INSTRUCTIONS>
+01:message/user[2]:
+    [01] <AGENTS_MD>
+    [02] <ENVIRONMENT_CONTEXT:cwd=<CWD>>
+02:message/developer:<realtime_conversation>\nRealtime conversation started.\n\nYou a...
+03:message/user:USER_ONE
+04:message/assistant:REMOTE_FIRST_REPLY
+
+## Remote Post-Compaction History Layout
+00:compaction:encrypted=true
+01:message/developer[2]:
+    [01] <PERMISSIONS_INSTRUCTIONS>
+    [02] <realtime_conversation>\nRealtime conversation ended.\n\nYou are...
+02:message/user[2]:
+    [01] <AGENTS_MD>
+    [02] <ENVIRONMENT_CONTEXT:cwd=<CWD>>
+03:message/user:USER_TWO

codex-rs/core/tests/suite/snapshots/all__suite__compact_remote__remote_pre_turn_compaction_restates_realtime_start_shapes.snap

@@ -0,0 +1,24 @@
+---
+source: core/tests/suite/compact_remote.rs
+expression: "format_labeled_requests_snapshot(\"Remote pre-turn auto-compaction while realtime remains active: compaction clears the reference baseline, so the follow-up request restates realtime-start instructions.\",\n&[(\"Remote Compaction Request\", &compact_request),\n(\"Remote Post-Compaction History Layout\", post_compact_request),])"
+---
+Scenario: Remote pre-turn auto-compaction while realtime remains active: compaction clears the reference baseline, so the follow-up request restates realtime-start instructions.
+
+## Remote Compaction Request
+00:message/developer:<PERMISSIONS_INSTRUCTIONS>
+01:message/user[2]:
+    [01] <AGENTS_MD>
+    [02] <ENVIRONMENT_CONTEXT:cwd=<CWD>>
+02:message/developer:<realtime_conversation>\nRealtime conversation started.\n\nYou a...
+03:message/user:USER_ONE
+04:message/assistant:REMOTE_FIRST_REPLY
+
+## Remote Post-Compaction History Layout
+00:compaction:encrypted=true
+01:message/developer[2]:
+    [01] <PERMISSIONS_INSTRUCTIONS>
+    [02] <realtime_conversation>\nRealtime conversation started.\n\nYou a...
+02:message/user[2]:
+    [01] <AGENTS_MD>
+    [02] <ENVIRONMENT_CONTEXT:cwd=<CWD>>
+03:message/user:USER_TWO