fix(app): guard against non-array diffs in session review

[jackblackbla]

Summary

• Add Array.isArray guard in event-reducer before passing diff data to reconcile(), preventing non-array values from being stored in session_diff
• Add safeDiffs() accessor in SessionReview component to normalize props.diffs, preventing .map() crash on non-array values
• Add 3 test cases for session.diff event handling: normal array, undefined, and non-array object payloads

Problem

Users are seeing a common crash in the session review UI:

TypeError: e.diffs.map is not a function
    at Object.fn (session-BdJIdAn0.js:19:50326)

The root cause is in event-reducer.ts where session.diff event properties are cast with as { sessionID: string; diff: FileDiff[] } — an unsafe cast with no runtime validation. If the SSE event delivers malformed diff data (e.g., undefined or an object instead of an array), reconcile() stores a non-array value in the session_diff store. The ?? [] fallback in the memo doesn't catch truthy non-array values like {}, so the SessionReview component crashes when calling .map() on it.

Test plan

• All 302 existing tests pass
• 3 new tests added for session.diff event edge cases (undefined, non-array)
• TypeScript typecheck passes across all 13 packages

🤖 Generated with Claude Code

[github-actions]

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[github-actions]

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[github-actions]

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

Potential Duplicate Found

PR #19221: fix(app): guard diffs with Array.isArray to prevent .map crash
#19221

Why it's related: This PR appears to address the exact same issue - preventing .map() crashes by guarding diffs with Array.isArray(). Both PRs are fixing the SessionReview component crash where non-array diffs cause TypeError: e.diffs.map is not a function. This is likely a duplicate of the same fix.

[jackblackbla]

Closing as duplicate of #19221 which covers the same fix with broader scope. Arrived at the same root cause independently — the unsafe cast + reconcile path in event-reducer.ts.