fix(app): guard diffs with Array.isArray to prevent .map crash

[ysm-dev]

Issue for this PR

Closes #19217

Type of change

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

What does this PR do?

props.diffs.map in SessionReview crashes when diffs is not an array. This happens when reopening old browser sessions because the SSE event reducer passes event.properties.diff directly to reconcile() without validating it's an array. If the data is malformed or missing, reconcile(undefined, { key: "file" }) can corrupt the store value into a non-array. The existing ?? [] fallback only catches null/undefined — a truthy non-array (like {}) passes through.

The fix adds Array.isArray guards at four points:

1. event-reducer.ts — validate props.diff before reconcile() (root cause fix, matches the pattern already used in sync.tsx)
2. session.tsx — guard session_diff[id] read with Array.isArray instead of ?? []
3. session.tsx — guard summary.diffs read with Array.isArray instead of ?? []
4. session-review.tsx — guard props.diffs access in the UI component as last-resort defense

How did you verify your code works?

• bun typecheck passes across all 13 packages
• Traced the full data flow from backend SSE events through reconcile() to the SessionReview component to confirm all paths now guarantee an array

Screenshots / recordings

N/A — no UI changes, this is a runtime crash fix.

Checklist

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

[ysm-dev]

The e2e (windows) failure is a pre-existing flake unrelated to this PR. The same tests fail on every other open PR right now (openai-websocket, fix-direct-route-sidebar-project-18943, gpt-5.4-prompt). All other checks pass (typecheck, unit tests, e2e linux, storybook, nix-eval).