fix(ui): render ready question marker in timeline

[Astro-Han]

Summary

Fixes a directly reported question sequencing regression with no separate issue: the question dock could appear while the timeline still lacked a visible handoff marker for the running question.

Why

The dock intentionally appears only after externalResultReady === true, but the timeline grouping layer originally filtered every pending or running question tool. The first fix allowed ready running questions to render, and this follow-up keeps that marker visible even when it follows a normal tool: ready running questions now split into their own single trow instead of being hidden inside a default-collapsed multi-tool trow.

Related Issue

None. This PR follows a direct maintainer report in Codex.

Human Review Status

Approved by @Astro-Han

Review Focus

• renderable() still keeps legacy pending and unready running questions hidden, but allows running questions with metadata.externalResultReady === true to reach the existing inline marker renderer.
• groupParts() now cuts a ready running question into its own trow, so the single-row auto-open path makes the marker visible without expanding unrelated tool details.
• The session-trow snap case exercises the production AssistantParts -> groupParts -> ToolPartDisplay path with a normal tool immediately before the ready question marker.

Risk Notes

No platform, persistence, dependency, permission, or migration surface is touched. This is a visible UI sequencing fix only.

How To Verify

Regression RED: packages/ui grouping test failed before the follow-up because the ready question still shared trow:t1 refs with the previous tool.
Diff check: git diff --check passed with no whitespace errors.
UI unit tests: packages/ui message-part grouping/tool/stale focused tests passed, 26 passed.
UI typecheck: cd packages/ui && bun run typecheck passed.
Dock predicate tests: cd packages/app && bun test --preload ./happydom.ts src/pages/session/blockers/use-session-blockers.test.ts passed, 14 passed.
App typecheck: cd packages/app && bun run typecheck passed.
Visual snap: bun run snap session-trow passed, including a normal tool immediately before the ready-question-marker case and a visible marker assertion.
Manual visual review: reviewed docs/design/preview/screenshots/session-trow.png and confirmed the ready-question-marker tile shows the preceding tool row plus the visible inline pending-question marker.

Screenshots or Recordings

bun run snap session-trow generated the local grid at docs/design/preview/screenshots/session-trow.png; the ready-question-marker tile was reviewed after the run.

Checklist

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• I linked the related issue, or stated in Summary why there is no issue.
• I described the review focus and any meaningful risks.
• I replaced the example block in How To Verify with the real verification steps and the key result for each.
• I did not introduce unrelated refactors, dependencies, generated files, or file changes beyond the stated scope.
• (conditional) I manually checked visible UI or copy changes when needed, with screenshots or recordings. Leave unticked only if no visible UI or copy changed.
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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro Plus

Run ID: aca5fafd-d1d6-4e4f-b401-28c98ea9f61a

📥 Commits

Reviewing files that changed from the base of the PR and between a660288 and 0aab4c9.

📒 Files selected for processing (6)

• packages/app/e2e/snap/fixtures/trow-snap-fixture-data.ts
• packages/app/e2e/snap/fixtures/trow-snap-fixture.tsx
• packages/app/e2e/snap/session-trow.snap.ts
• packages/ui/src/components/message-part-stale.test.ts
• packages/ui/src/components/message-part/grouping.test.ts
• packages/ui/src/components/message-part/grouping.ts

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/question-marker-ready

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[github-actions]

Suggested priority: P2 (includes non-doc, non-test paths outside the low-risk bucket).

P1/P0 are reserved for maintainer confirmation. Please relabel manually if this is a release blocker, security issue, data-loss risk, or updater/runtime failure.