fix(pulls): report the real reason when a merge is refused (#1107)

## Problem

`tea pr merge <index>` reports the same misleading error for every refusal:

```
failed to merge PR, is it still open?
```

The PR usually *is* still open — `tea pr <index>` shows it as open and lists `Conflicting files` — so the message sends users looking in the wrong direction.

## Root cause

Gitea answers an unmergeable PR with a 405 and a body naming the actual cause. The SDK's `MergePullRequest` is built on `getStatusCode`, which returns only the status code and never calls `statusCodeToErr`, so the body is discarded. tea receives `success=false, err=nil` with no server explanation to pass on, and fell back to guessing that the PR might be closed.

## Changes

- Derive the refusal reason from the pull request when a merge fails: already merged, closed, draft, or not mergeable.
- When the PR looks mergeable but was refused anyway, name the conditions tea cannot observe (required status checks, requested reviews, branch protection) instead of guessing.
- Include the PR index in the error.
- Add table-driven tests for every reason, plus the case where the follow-up PR lookup fails.

The extra API call happens only on the failure path.

Fixes #1022

---------

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Reviewed-on: https://gitea.com/gitea/tea/pulls/1107
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Jan Baer <jan.s.baer@googlemail.com>

modules/task/pull_merge.go

@@ -19,8 +19,36 @@ func PullMerge(requestCtx stdctx.Context, login *config.Login, repoOwner, repoNa
 	if err != nil {
 		return err
 	}
-	if !success {
-		return fmt.Errorf("failed to merge PR, is it still open?")
+	if success {
+		return nil
+	}
+	return fmt.Errorf("failed to merge PR #%d: %s", index,
+		mergeFailureReason(requestCtx, client, repoOwner, repoName, index))
+}
+
+// mergeFailureReason returns why merging was refused. The SDK reports refusal as
+// success=false and discards Gitea's explanatory body, so the reason has to be
+// re-derived from the PR. Costs one API call, on the failure path only.
+func mergeFailureReason(requestCtx stdctx.Context, client *gitea.Client, repoOwner, repoName string, index int64) string {
+	// Fallback naming the conditions tea cannot observe, used when the PR looks
+	// mergeable but the merge was refused anyway.
+	const refused = "the server refused the merge; check required status checks, requested reviews, or branch protection rules"
+
+	pr, _, err := client.PullRequests.GetPullRequest(requestCtx, repoOwner, repoName, index)
+	if err != nil || pr == nil {
+		return refused
+	}
+
+	switch {
+	case pr.HasMerged:
+		return "it has already been merged"
+	case pr.State == gitea.StateClosed:
+		return "it is closed"
+	case pr.Draft:
+		return "it is a draft; mark it ready for review first"
+	case !pr.Mergeable:
+		return "it has conflicting files or is otherwise not mergeable"
+	default:
+		return refused
 	}
-	return nil
 }