fix: register gitea_lock_issue as MCP tool (Closes #521)

Move @mcp.tool() from internal _list_open_pulls helper onto
gitea_lock_issue so author sessions can discover and call the
issue lock required by gitea_create_pr.

Add regression tests for MCP registration and create_pr lock flow.

gitea_mcp_server.py

@@ -1446,7 +1446,6 @@ def gitea_create_issue(
     return _with_optional_url({"number": data["number"]}, data.get("html_url"))
 
 
-@mcp.tool()
 def _list_open_pulls(h: str, o: str, r: str, auth: str) -> list[dict]:
     """Fetch all OPEN pull requests for a repo (used for stacked-base proof, #484)."""
     try:
@@ -1457,6 +1456,7 @@ def _list_open_pulls(h: str, o: str, r: str, auth: str) -> list[dict]:
         )
 
 
+@mcp.tool()
 def gitea_lock_issue(
     issue_number: int,
     branch_name: str,