test(session): regression test for #26574 + mirror loosening on Vcs.FileDiff

[kitlangton]

Follow-up to #26574 (already merged).

What this adds

1. Regression test (packages/opencode/test/server/session-diff-missing-patch.test.ts). fix(sessions): allow optional patch field in diff for migrated sessions #26574 shipped without a test; this exercises the actual bug shape:

   • Create a session
   • Write a session_diff storage entry with no patch field (mimics legacy/imported on-disk shape)
   • Call `GET /session//diff`
   • Pre-fix the typed response encoder rejects → 400 → desktop session load fails
   • Post-fix the response is 200 with `patch: undefined` in the entry

   Verified red → green → red → green locally before pushing.

2. Mirror schema loosening on `Vcs.FileDiff` (packages/opencode/src/project/vcs.ts:231). Same shape (`file/patch/additions/deletions/status`), used by /instance/vcs/diff. Current producer always sets `patch`, but loosening matches `Snapshot.FileDiff` so any future producer that omits patch can't reintroduce the same crash on the sibling endpoint.

3. Comment on `Snapshot.FileDiff.patch` explaining why it's optional, so the next reader doesn't tighten it back.

Why this matters

Same disease family as #26546, #26553, #26566, #26572: post-Hono-migration strict Schema validation rejects legitimate-looking on-disk data, the response 4xx's, and the client crashes (or is told nothing useful). The original PR fixed the symptom on one endpoint without nailing it down with a test or extending the fix to the sibling.

Test

• `bun run test test/server/session-diff-missing-patch.test.ts` — passes (would fail on origin before fix(sessions): allow optional patch field in diff for migrated sessions #26574)