Clarify realtime v2 context and handoff messages (#17896)

## Summary
- wrap realtime startup context in
`<startup_context>...</startup_context>` tags
- prefix V2 mirrored user text and relayed backend text with `[USER]` /
`[BACKEND]`
- remove the V2 progress suffix and replace the final V2 handoff output
with a short completion acknowledgement while preserving the existing V1
wrapper

## Testing
- cargo test -p codex-api
realtime_v2_session_update_includes_background_agent_tool_and_handoff_output_item
-- --exact
- cargo test -p codex-app-server webrtc_v2_background_agent_
- cargo test -p codex-app-server webrtc_v2_text_input_is_
- cargo test -p codex-core conversation_user_text_turn_is_

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

@@ -9,4 +9,4 @@ content[0].type: input_text
 model_has_full_user_text: true
 realtime_text_equal_full_user_text: false
 realtime_text_approx_tokens: 300
-content[0].text: mirror-head alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alph…2417 tokens truncated…ega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega  mirror-tail
+content[0].text: [USER] mirror-head alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alpha alp…2419 tokens truncated…ega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega omega  mirror-tail

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

@@ -6,5 +6,5 @@ type: conversation.item.create
 item.type: message
 item.role: user
 content[0].type: input_text
-content[0].text: typed follow-up for realtime
+content[0].text: [USER] typed follow-up for realtime
 response.create: false