fix(core): compact on request byte envelope

[fengjikui]

Issue for this PR

Closes #35013

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Adds an optional compaction.max_request_bytes guard for proactive compaction. The existing token-based behavior remains unchanged unless the setting is configured; when it is configured, the serialized request envelope can trigger compaction before a provider rejects the request for size.

This keeps the fallback conservative for providers/routes that can hit request-size limits before the token context threshold.

How did you verify your code works?

• bun typecheck from packages/core
• bun test --timeout 30000 test/session-compaction.test.ts test/config/config.test.ts from packages/core
• bun test --timeout 30000 test/session-runner.test.ts --grep 'compaction|overflow' from packages/core
• bunx prettier --check packages/core/src/config/compaction.ts packages/core/src/session/compaction.ts packages/core/test/config/config.test.ts packages/core/test/session-compaction.test.ts
• git diff --check
• pre-push bun turbo typecheck completed successfully: 30/30 tasks

Screenshots / recordings

N/A

Checklist

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

[github-actions]

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

Potential Related PRs Found

Based on the search, I found several related PRs that involve compaction and request sizing in the core package:

1. fix(core): estimate semantic request context - Recent PR addressing request context estimation, likely related to the same issue space

2. fix: bound compaction request size - Directly related to bounding request size during compaction

3. fix(core): guard non-reducing compaction - Related compaction guard mechanism

These PRs are related to compaction and request size management, which is the core concern of the current PR (#35687). However, they don't appear to be duplicates—rather, they're part of an evolving compaction strategy for handling request size limits.

[github-actions]

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