fix(core): consolidate diff size constants into packages/core/src/dif…

[qian-jiahong]

Issue for this PR

Closes #31655 (fix: #31655)

Type of change

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

What does this PR do?

• Three-layer defense: generation (snapshot), dedicated endpoint (summary), messages serialization (message-v2)
• 100KB threshold for messages API (batch returns all diffs at once) vs 1MB for per-file diff endpoints (one file at a time) — conservative at batch API to prevent frontend freeze
• Frontend guard (MAX_RECONSTRUCT_BYTES) as defense-in-depth: skip expensive content reconstruction + re-diff, fall through to fast unified-diff parser
• Pre-check file content size in snapshot before attempting Number.MAX_SAFE_INTEGER to avoid wasted Myers diff on large files

How did you verify your code works?

1. Reproduced the freeze: Opened a project with a stalled session containing large diffs (multi-MB patches). Confirmed renderer freeze on fileDiffFromPatch → completePatchContents → parseDiffFromFile path.
2. Applied fixes and rebuilt: Ran electron-vite build in packages/desktop to produce a production build with all four fix layers.
3. Confirmed the fix: Reopened the same project. The session loaded without freeze. Large patches were:
   Capped at 1MB during snapshot generation (snapshot/index.ts)
   Discarded >1MB at the per-file diff endpoint (summary.ts)
   Stripped >100KB from messages API responses (message-v2.ts)
   Skipped content reconstruction on the frontend for patches >100KB, falling through to the fast parsePatchFiles path (session-diff.ts)
4. Verified diff display still works: Normal-sized diffs continued to render correctly with full before/after content reconstruction.
5. Checked no regression in snapshot generation: Ran existing snapshot tests via bun test in packages/opencode.

Screenshots / recordings

If this is a UI change, please include a screenshot or recording.

Checklist

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

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

[qian-jiahong]

Issue for this PR

Closes #31655 (fix: #31655)

Type of change

* [x]  Bug fix

* [ ]  New feature

* [ ]  Refactor / code improvement

* [ ]  Documentation

What does this PR do?

* Three-layer defense: generation (snapshot), dedicated endpoint (summary), messages serialization (message-v2)

* 100KB threshold for messages API (batch returns all diffs at once) vs 1MB for per-file diff endpoints (one file at a time) — conservative at batch API to prevent frontend freeze

* Frontend guard (`MAX_RECONSTRUCT_BYTES`) as defense-in-depth: skip expensive content reconstruction + re-diff, fall through to fast unified-diff parser

* Pre-check file content size in snapshot before attempting `Number.MAX_SAFE_INTEGER` to avoid wasted Myers diff on large files

How did you verify your code works?

Screenshots / recordings

If this is a UI change, please include a screenshot or recording.

Checklist

* [x]  I have tested my changes locally

* [x]  I have not included unrelated changes in this PR

verify :

1. Reproduced the freeze: Opened a project with a stalled session containing large diffs (multi-MB patches). Confirmed renderer freeze on fileDiffFromPatch → completePatchContents → parseDiffFromFile path.

2. Applied fixes and rebuilt: Ran electron-vite build in packages/desktop to produce a production build with all four fix layers.

3. Confirmed the fix: Reopened the same project. The session loaded without freeze. Large patches were:
   Capped at 1MB during snapshot generation (snapshot/index.ts)
   Discarded >1MB at the per-file diff endpoint (summary.ts)
   Stripped >100KB from messages API responses (message-v2.ts)
   Skipped content reconstruction on the frontend for patches >100KB, falling through to the fast parsePatchFiles path (session-diff.ts)

4. Verified diff display still works: Normal-sized diffs continued to render correctly with full before/after content reconstruction.

5. Checked no regression in snapshot generation: Ran existing snapshot tests via bun test in packages/opencode.

This PR doesn't fully meet our contributing guidelines and PR template.

What needs to be fixed:

* "How did you verify your code works?" section is empty. Please explain how you tested.

Please edit this PR description to address the above within 2 hours, or it will be automatically closed.

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updated.

[github-actions]

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

Found potential related PRs:

1. fix(ui): avoid blocking on large file diffs (PR fix(ui): avoid blocking on large file diffs #30962)

   • Related: Addresses renderer freezing issues when handling large file diffs

2. fix(ui): prepare diffs off the render thread (PR fix(ui): prepare diffs off the render thread #31309)

   • Related: Also tackles performance issues with diff processing to prevent blocking the render thread

These PRs are related to the same problem domain (preventing renderer freeze on large diffs), though PR #31928 takes a different approach by consolidating diff size constants and implementing a three-layer defense strategy at the core/API level rather than just UI optimization.