fix(pr_merge): send Gitea's merge_title_field; merge_message_title was silently ignored (gitea-mcp#77)

Gitea 1.27's MergePullRequestOption has merge_title_field, not merge_message_title; an unknown field is
dropped without an error, so a custom merge title never took effect. The tool's argument keeps its name.
The new test checks the field name on the wire (the existing fake never did).

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SZkhi49hfeSqBp5aZJNXsM

internal/gitea/pulls.go

@@ -107,7 +107,7 @@ func (c *Client) GetPullRequestDiff(ctx context.Context, owner, repo string, ind
 
 type MergePRArgs struct {
 	Do    string `json:"Do"`
-	Title string `json:"merge_message_title,omitempty"`
+	Title string `json:"merge_title_field,omitempty"`
 	Body  string `json:"merge_message_field,omitempty"`
 	// HeadCommitID binds the merge to the commit whose CI was checked: if the PR branch moved, Gitea refuses.
 	HeadCommitID string `json:"head_commit_id,omitempty"`