fix(prompt): keep plan/build reminders stable

[namnguyen0503]

Issue for this PR

Closes #26749

Type of change

• Bug fix

What does this PR do?

Fixes prompt-history instability where Plan/Build synthetic <system-reminder> content could move between historical user turns across later request serializations.

Root cause: Plan/Build reminders were attached to the latest user turn, and one later path mutated existing message parts in place. This allowed a historical user message to serialize once as Continue plus a Plan -> Build reminder, then later serialize as only Continue.

Changes:

• Anchor Plan/Build reminders to a stable turn boundary.
• Clone/reconstruct messages and parts instead of mutating existing history objects.
• Keep the later follow-up reminder rewrite immutable.
• Add regression coverage for Plan -> Build -> Continue -> later Build turn.

This preserves prefix-cache stability for OpenAI-compatible local backends such as llama.cpp, vLLM, and LM Studio because previously seen prompt messages remain byte-stable.

How did you verify your code works?

Ran:

• bun test test/session/prompt.test.ts -t "keeps plan/build system reminders anchored to the original user turn across later serializations"
• bun test test/session/prompt.test.ts -t "static loop consumes queued replies across turns"
• bun test test/session/prompt.test.ts
• bun turbo typecheck

Result: all passed.

I also ran a source-checkout smoke test using the repository's fake OpenAI-compatible TestLLMServer, without loading a real model.

The smoke test reproduced Plan -> Build -> Continue -> later Build and verified that the historical Continue + <system-reminder> message stayed byte-stable across later serializations.

Observed:

• continueIndex: 3
• firstDiffMsg: 4

Screenshots / recordings

Not applicable. This is a prompt serialization / cache stability bug fix with regression tests.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

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[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Potential Duplicate Found

PR #26743: fix(session): persist plan mode reminders

• fix(session): persist plan mode reminders #26743

This PR appears related as it also addresses plan mode reminders. While the current PR (27863) focuses on keeping Plan/Build reminders stable across serializations to preserve prefix-cache stability, PR #26743 addresses persisting plan mode reminders. These may be related efforts around plan mode reminder handling and should be reviewed to ensure they don't conflict or duplicate work.

[namnguyen0503]

Closing and recreating with the exact PR template from the start to avoid stale compliance labels.