fix(prompt): keep plan/build reminders stable

[namnguyen0503]

What does this PR do?

Fixes #26749.

This 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

This means the first changed message was at the new append boundary, while the previous Continue message containing the Plan -> Build reminder stayed identical.

Screenshots / recordings

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

[github-actions]

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[namnguyen0503]

Updated the PR description to use the required template sections and added Fixes #26749.

[namnguyen0503]

The PR body now follows the required template and includes Fixes #26749, but the needs:issue / needs:compliance labels are still present. Could you re-run or clear the compliance check?

[namnguyen0503]

All PR checks are now passing, including pr-standards/check-compliance. The stale needs:compliance label still appears to be present; could a maintainer clear it or re-run the label cleanup?

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