fix(mcp): fail closed when merger lease cannot resolve live PR head

gitea_acquire_merger_pr_lease previously continued when GET /pulls returned
an empty head SHA. Fail closed so exact-head scoping never proceeds with an
unresolvable live head. Add regression coverage for empty head payloads.

Fixes #718
Refs #723

gitea_mcp_server.py

@@ -10821,7 +10821,20 @@ def gitea_acquire_merger_pr_lease(
         or ""
     )
     live_head = str(live_head).strip()
-    if live_head and live_head != head_pin:
+    # Fail closed when live PR head cannot be resolved — never proceed with
+    # an unpinned/unknown head even if candidate_head was supplied (#718).
+    if not live_head:
+        return {
+            "success": False,
+            "acquired": False,
+            "reasons": [
+                "live PR head could not be resolved from GET /pulls response "
+                "(exact-head scoping; fail closed)"
+            ],
+            "live_head": None,
+            "candidate_head": head_pin,
+        }
+    if live_head != head_pin:
         return {
             "success": False,
             "acquired": False,