Unify remote compaction snapshot mocks around default endpoint behavior (#12050)

## Summary
- standardize remote compaction test mocking around one default behavior
in shared helpers
- make default remote compact mocks mirror production shape: keep
`message/user` + `message/developer`, drop assistant/tool artifacts,
then append a summary user message
- switch non-special `compact_remote` tests to the shared default mock
instead of ad-hoc JSON payloads

## Special-case tests that still use explicit mocks
- remote compaction error payload / HTTP failure behavior
- summary-only compact output behavior
- manual `/compact` with no prior user messages
- stale developer-instruction injection coverage

## Why
This removes inconsistent manual remote compaction fixtures and gives us
one source of truth for normal remote compact behavior, while preserving
explicit mocks only where tests intentionally cover non-default
behavior.

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

@@ -0,0 +1,32 @@
+---
+source: core/tests/suite/compact.rs
+assertion_line: 3152
+expression: "format_labeled_requests_snapshot(\"Pre-turn compaction during model switch (without pre-sampling model-switch compaction): current behavior strips incoming <model_switch> from the compact request and restores it in the post-compaction follow-up request.\",\n&[(\"Initial Request (Previous Model)\", &requests[0]),\n(\"Local Compaction Request\", &requests[1]),\n(\"Local Post-Compaction History Layout\", &requests[2]),])"
+---
+Scenario: Pre-turn compaction during model switch (without pre-sampling model-switch compaction): current behavior strips incoming <model_switch> from the compact request and restores it in the post-compaction follow-up request.
+
+## Initial Request (Previous Model)
+00:message/developer:<PERMISSIONS_INSTRUCTIONS>
+01:message/user:<AGENTS_MD>
+02:message/user:<ENVIRONMENT_CONTEXT:cwd=<CWD>>
+03:message/developer:<PERMISSIONS_INSTRUCTIONS>
+04:message/user:BEFORE_SWITCH_USER
+
+## Local Compaction Request
+00:message/developer:<PERMISSIONS_INSTRUCTIONS>
+01:message/user:<AGENTS_MD>
+02:message/user:<ENVIRONMENT_CONTEXT:cwd=<CWD>>
+03:message/developer:<PERMISSIONS_INSTRUCTIONS>
+04:message/user:BEFORE_SWITCH_USER
+05:message/assistant:BEFORE_SWITCH_REPLY
+06:message/user:<SUMMARIZATION_PROMPT>
+
+## Local Post-Compaction History Layout
+00:message/developer:<PERMISSIONS_INSTRUCTIONS>
+01:message/developer:<personality_spec> The user has requested a new communication st...
+02:message/user:<AGENTS_MD>
+03:message/user:<ENVIRONMENT_CONTEXT:cwd=<CWD>>
+04:message/user:BEFORE_SWITCH_USER
+05:message/user:<COMPACTION_SUMMARY>\nPRETURN_SWITCH_SUMMARY
+06:message/developer:<model_switch>\nThe user was previously using a different model....
+07:message/user:AFTER_SWITCH_USER

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

@@ -0,0 +1,21 @@
+---
+source: core/tests/suite/compact_remote.rs
+expression: "format_labeled_requests_snapshot(\"Remote mid-turn compaction after an earlier summary compaction: the older summary remains in model-visible history and round-trips into the next compact request.\",\n&[(\"Second Turn Request (Before Mid-Turn Compaction)\", &requests[1]),\n(\"Remote Compaction Request\", &compact_request),])"
+---
+Scenario: Remote mid-turn compaction after an earlier summary compaction: the older summary remains in model-visible history and round-trips into the next compact request.
+
+## Second Turn Request (Before Mid-Turn Compaction)
+00:message/user:USER_ONE
+01:message/user:<COMPACTION_SUMMARY>\nREMOTE_OLDER_SUMMARY
+02:message/developer:<PERMISSIONS_INSTRUCTIONS>
+03:message/user:<AGENTS_MD>
+04:message/user:<ENVIRONMENT_CONTEXT:cwd=<CWD>>
+05:message/user:<COMPACTION_SUMMARY>\nREMOTE_LATEST_SUMMARY
+06:message/user:USER_TWO
+
+## Remote Compaction Request
+00:message/user:USER_ONE
+01:message/developer:<PERMISSIONS_INSTRUCTIONS>
+02:message/user:<AGENTS_MD>
+03:message/user:<ENVIRONMENT_CONTEXT:cwd=<CWD>>
+04:message/user:<COMPACTION_SUMMARY>\nREMOTE_OLDER_SUMMARY

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

@@ -0,0 +1,19 @@
+---
+source: core/tests/suite/compact_remote.rs
+expression: "format_labeled_requests_snapshot(\"Remote mid-turn compaction where compact output has only summary user content: continuation layout reinjects canonical context before that summary.\",\n&[(\"Remote Compaction Request\", &compact_request),\n(\"Remote Post-Compaction History Layout\", &requests[1]),])"
+---
+Scenario: Remote mid-turn compaction where compact output has only summary user content: continuation layout reinjects canonical context before that summary.
+
+## Remote Compaction Request
+00:message/developer:<PERMISSIONS_INSTRUCTIONS>
+01:message/user:<AGENTS_MD>
+02:message/user:<ENVIRONMENT_CONTEXT:cwd=<CWD>>
+03:message/user:USER_ONE
+04:function_call/test_tool
+05:function_call_output:unsupported call: test_tool
+
+## Remote Post-Compaction History Layout
+00:message/developer:<PERMISSIONS_INSTRUCTIONS>
+01:message/user:<AGENTS_MD>
+02:message/user:<ENVIRONMENT_CONTEXT:cwd=<CWD>>
+03:message/user:<COMPACTION_SUMMARY>\nREMOTE_SUMMARY_ONLY

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

@@ -0,0 +1,28 @@
+---
+source: core/tests/suite/compact_remote.rs
+expression: "format_labeled_requests_snapshot(\"Remote pre-turn compaction during model switch currently excludes incoming user input, strips incoming <model_switch> from the compact request payload, and restores it in the post-compaction follow-up request.\",\n&[(\"Initial Request (Previous Model)\", &requests[0]),\n(\"Remote Compaction Request\", &compact_request),\n(\"Remote Post-Compaction History Layout\", &requests[1]),])"
+---
+Scenario: Remote pre-turn compaction during model switch currently excludes incoming user input, strips incoming <model_switch> from the compact request payload, and restores it in the post-compaction follow-up request.
+
+## Initial Request (Previous Model)
+00:message/developer:<PERMISSIONS_INSTRUCTIONS>
+01:message/user:<AGENTS_MD>
+02:message/user:<ENVIRONMENT_CONTEXT:cwd=<CWD>>
+03:message/user:BEFORE_SWITCH_USER
+
+## Remote Compaction Request
+00:message/developer:<PERMISSIONS_INSTRUCTIONS>
+01:message/user:<AGENTS_MD>
+02:message/user:<ENVIRONMENT_CONTEXT:cwd=<CWD>>
+03:message/user:BEFORE_SWITCH_USER
+04:message/assistant:BEFORE_SWITCH_REPLY
+
+## Remote Post-Compaction History Layout
+00:message/user:BEFORE_SWITCH_USER
+01:message/developer:<PERMISSIONS_INSTRUCTIONS>
+02:message/developer:<personality_spec> The user has requested a new communication st...
+03:message/user:<AGENTS_MD>
+04:message/user:<ENVIRONMENT_CONTEXT:cwd=<CWD>>
+05:message/user:<COMPACTION_SUMMARY>\nREMOTE_SWITCH_SUMMARY
+06:message/developer:<model_switch>\nThe user was previously using a different model....
+07:message/user:AFTER_SWITCH_USER