## 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>
