fix: inject duplicate-work context fetcher for testable lock/create_pr paths (#400)

Expose issue_duplicate_context_fetcher on the MCP server so lock_issue,
commit_files, and create_pr duplicate rechecks avoid live Gitea calls in
unit tests. Update affected test suites to patch the fetcher without
weakening duplicate-work gate assertions.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

gitea_mcp_server.py

@@ -548,7 +548,7 @@ def _load_existing_issue_lock() -> dict | None:
     if not os.path.exists(ISSUE_LOCK_FILE):
         return None
     try:
-        with open(ISSUE_LOCK_FILE, encoding="utf-8") as f:
+        with open(ISSUE_LOCK_FILE, "r", encoding="utf-8") as f:
             data = json.load(f)
         return data if isinstance(data, dict) else None
     except Exception:
@@ -642,7 +642,7 @@ def _branch_entry_name(branch: dict | str) -> str:
     return str(branch.get("name") or branch.get("ref") or "")
 
 
-def _collect_issue_duplicate_context(
+def _live_fetch_issue_duplicate_context(
     h: str,
     o: str,
     r: str,
@@ -667,6 +667,22 @@ def _collect_issue_duplicate_context(
     return open_prs, branch_names, claim_entry
 
 
+# Injectable duplicate-work context fetcher (#400). Production uses the live
+# Gitea API path above; unit tests patch this symbol instead of hitting the
+# network.
+issue_duplicate_context_fetcher = _live_fetch_issue_duplicate_context
+
+
+def _collect_issue_duplicate_context(
+    h: str,
+    o: str,
+    r: str,
+    auth: str,
+    issue_number: int,
+) -> tuple[list[dict], list[str], dict]:
+    return issue_duplicate_context_fetcher(h, o, r, auth, issue_number)
+
+
 def _assess_issue_duplicate_gate(
     issue_number: int,
     *,