fix(session): preserve live tail messages

[src-opn]

Summary

• Always merge live-only transcript tail messages with the latest snapshot so newly sent turns stay visible even if the snapshot endpoint lags after session.idle.
• Removes the streaming-only gate from rendered session message derivation.
• Adds regression coverage for the stale-snapshot/live-tail race.

Closes #1747

Evidence

Build verification

• bun test apps/app/scripts/session-render-state.test.ts -- passed: 8 tests, 12 assertions.
• pnpm --filter @openwork/app build -- passed.
• pnpm --filter @openwork/app typecheck -- failed on existing unrelated TypeScript errors in connection/settings/desktop files; no errors were reported in the changed session files.

API verification

• No API changes.

UI verification

• No screenshot captured; this is a session render-state race covered by the focused regression test.

Test instructions

1. Open an existing chat thread.
2. Send several messages until the backend has a stale snapshot while live SSE has the newest tail.
3. Verify the last user/assistant turn remains visible after the run becomes idle, without switching threads.

[vercel]

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Project	Deployment	Actions	Updated (UTC)
openwork-app	Ready	Preview, Comment	May 11, 2026 4:29pm
openwork-den	Ready	Preview, Comment	May 11, 2026 4:29pm
openwork-den-worker-proxy	Ready	Preview, Comment	May 11, 2026 4:29pm
openwork-landing	Ready	Preview, Comment, Open in v0	May 11, 2026 4:29pm
openwork-share	Ready	Preview, Comment	May 11, 2026 4:29pm

[src-opn]

Verification

• bun test apps/app/scripts/session-render-state.test.ts passed: 8 tests, 12 assertions.
• pnpm --filter @openwork/app build passed.
• pnpm --filter @openwork/app typecheck failed before completion on existing unrelated TypeScript errors in connection/settings/desktop code; no reported errors were in the changed session render files.

This PR specifically covers the race where live transcript messages contain the latest tail while the snapshot still only contains earlier history.

[src-opn]

Rebase Verification

Rebased this PR onto latest origin/dev and resolved the conflict in session-surface.tsx by keeping the updated dev behavior while relying on deriveRenderedSessionMessages to always preserve live-only tail messages.

Verification after rebase:

• bun test apps/app/scripts/session-render-state.test.ts passed: 8 tests, 12 assertions.
• pnpm --filter @openwork/app build passed.