fix(app): recover deleted session sync races

[YOMXXX]

Issue for this PR

Closes #29323

Type of change

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

What does this PR do?

Handles a deleted-session race in the app directory sync layer. When a stale sync, diff, todo, or history request receives the SDK-wrapped session-not-found 404 after a session has been deleted, directory sync now removes the local session state/caches and resolves instead of surfacing the stale request error to the UI. Other 404s and non-session errors still rethrow.

How did you verify your code works?

• cd packages/app && PATH="$HOME/.bun/bin:$PATH" bun test src/context/directory-sync-error.test.ts
• cd packages/app && PATH="$HOME/.bun/bin:$PATH" bun typecheck
• cd packages/app && PATH="$HOME/.bun/bin:$PATH" bun test --preload ./happydom.ts ./src
• git diff --check
• PATH="$HOME/.bun/bin:$PATH" .husky/pre-push

Screenshots / recordings

Not applicable; error handling fix.

Checklist

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

[ghost]

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

Potential duplicate found:

• PR fix(app): handle deleted session sync cache in V2 titlebar #29328 - fix(app): handle deleted session sync cache
  • fix(app): handle deleted session sync cache in V2 titlebar #29328
  • Why it's related: This PR directly addresses the same issue (deleted session handling in the sync layer). Both PRs appear to be fixing related problems with session deletion and sync cache management in the app directory.