fix(web): say why a merge was refused

[TonybynMp4]

Closes #13065.

When a merge is refused, the server drops gh's error output, so the toast always says to check write access, checks and conflicts. On a branch that only needs a review, all three are fine, and the real cause never shows up in T3 Code.

The failed-merge toast now picks its hint from state the panel already holds, checking in this order:

1. The branch conflicts with the base.
2. The host reports a required review.
3. A reviewer requested changes.
4. Checks are failing or still running.
5. The host says the pull request is mergeable, yet the merge was refused. The toast blames the branch rules. This also covers a self-approval, which the panel shows as "approved" but no rule counts.

If mergeability is unknown, the toast keeps today's generic text. The change sends no new requests and doesn't touch the contracts. Mobile has no merge action, so only the web panel changes.

Tests are in pullRequestDetail.logic.test.ts.

Screenshots

Before:

After:

Summary by CodeRabbit

Bug Fixes

• Improved merge failure messages by identifying likely blockers, including unresolved conflicts, required reviews, requested changes, failing or pending checks, and branch protection rules.
• Preserved specific guidance for rebase failures.
• Retained a general fallback message when merge status or the underlying blocker cannot be determined.

[macroscopeapp]

Approvability

Verdict: Approved at bb8b619

Macroscope's review found this PR approvable — This is a focused bug fix that improves existing merge-failure messaging using state already loaded by the web panel. It adds no new requests, data contracts, permissions, or production configuration, and the decision logic is covered by unit tests.

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ℹ️ Recent review info

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Reviewing files that changed from the base of the PR and between ae02008 and bb8b619.

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• apps/web/src/components/pullRequest/pullRequestDetail.logic.ts

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📝 Walkthrough

Walkthrough

The merge failure flow now selects a specific refusal message from mergeability, review status, and check status. The detail panel uses this message for failed merges and retains the generic fallback when the state does not identify a blocker. Tests cover the selection order and fallback.

Changes

Merge refusal hints

Layer / File(s)	Summary
Merge blocker hint derivation apps/web/src/components/pullRequest/pullRequestDetail.logic.ts, apps/web/src/components/pullRequest/pullRequestDetail.logic.test.ts	mergeRefusalHint maps conflicts, review requirements, check states, and branch rules to refusal messages. Tests cover precedence and unknown mergeability.
Detail panel merge failure integration apps/web/src/components/pullRequest/PullRequestDetailPanel.tsx	Failed merge actions use mergeRefusalHint and fall back to the existing generic merge hint when needed.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix · Severity of issue fixed: Low

Suggested reviewers: bil0000

Merge Risk: ⚪ Minimal · up to bb8b6

The merge panel now explains common refusal causes while preserving the generic message when state is inconclusive. No actionable merge-blocking risk remains.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 66.67% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 3 functions across 3 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: explaining why a merge was refused.
Description check	✅ Passed	The description explains what changed, why it changed, the decision order, scope, tests, and UI results with before/after screenshots. It does not use the template's exact "UI Changes" heading and omi…
Linked Issues check	✅ Passed	The changes satisfy coding requirements in #13065. mergeRefusalHint uses existing mergeability, reviewDecision, and checksState. It reports a missing review and checks other known refusal caus…
Out of Scope Changes check	✅ Passed	The changes stay within #13065. They modify the web pull request detail logic, merge-failure integration, and focused automated tests. These changes support the requested refusal message. No unrelated…

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Inline comments:
In `@apps/web/src/components/pullRequest/pullRequestDetail.logic.ts`:
- Line 1089: Update mergeRefusalHint to return null immediately when
state.mergeability is "unknown", before evaluating reviewDecision or
checksState; preserve the existing handling for conflicting and other known
mergeability states.
- Line 1105: Update the mergeRefusalHint message to say checks may be “still
running or awaiting action,” covering workflow checks converted from
“action-required” to “pending” while preserving the existing branch-rules
wording.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Repository: pingdotgg/t3code/.coderabbit.yaml

Review profile: CHILL

Plan: Advanced

Run ID: 0387cde6-5964-4148-b338-3dcc93f77c5f

📥 Commits

Reviewing files that changed from the base of the PR and between aff9318 and ae02008.

📒 Files selected for processing (3)

• apps/web/src/components/pullRequest/PullRequestDetailPanel.tsx
• apps/web/src/components/pullRequest/pullRequestDetail.logic.test.ts
• apps/web/src/components/pullRequest/pullRequestDetail.logic.ts

Included review availability: Your plan provides up to 10 included reviews per hour; 9 remain after this review.

Dismissing prior approval to re-evaluate bb8b619