fix: return structured conflict-fix push assessment errors (Closes #519)

Resolve PR via pulls API before fetching lease comments so
gitea_assess_conflict_fix_push fails closed with actionable reasons
instead of surfacing HTTP 500 when Gitea returns not-found errors.

Add regression tests and document assessment-failure handoff in work-issue.

gitea_mcp_server.py

@@ -2650,6 +2650,85 @@ def gitea_get_pr_review_feedback(
     }
 
 
+def _fetch_pr_lease_comments_safe(
+    pr_number: int,
+    *,
+    remote: str,
+    host: str | None,
+    org: str | None,
+    repo: str | None,
+    limit: int = 100,
+    require_open: bool = False,
+) -> dict:
+    """Fetch PR thread comments with structured fail-closed errors (#519)."""
+    h, o, r = _resolve(remote, host, org, repo)
+    auth = _auth(h)
+    resolved_repo = f"{o}/{r}"
+    pr_url = f"{repo_api_url(h, o, r)}/pulls/{pr_number}"
+    try:
+        pr = api_request("GET", pr_url, auth)
+    except RuntimeError as exc:
+        return {
+            "success": False,
+            "comments": [],
+            "reasons": [
+                "PR lookup failed before conflict-fix push assessment "
+                f"(pr_number={pr_number}, repo={resolved_repo}, remote={remote}): "
+                f"{_redact(str(exc))}"
+            ],
+            "pr_lookup": "failed",
+            "resolved_repo": resolved_repo,
+            "remote": remote,
+            "pr_number": pr_number,
+        }
+    pr_state = (pr.get("state") or "").strip().lower()
+    if require_open and pr_state != "open":
+        return {
+            "success": False,
+            "comments": [],
+            "reasons": [
+                f"PR #{pr_number} on {resolved_repo} is not open "
+                f"(state={pr_state or 'unknown'})"
+            ],
+            "pr_lookup": "not_open",
+            "resolved_repo": resolved_repo,
+            "remote": remote,
+            "pr_number": pr_number,
+            "head_sha": (pr.get("head") or {}).get("sha"),
+        }
+    api = f"{repo_api_url(h, o, r)}/issues/{pr_number}/comments"
+    try:
+        comments = api_request("GET", api, auth)
+    except RuntimeError as exc:
+        return {
+            "success": False,
+            "comments": [],
+            "reasons": [
+                "PR comment fetch failed during conflict-fix push assessment "
+                f"(pr_number={pr_number}, issue_index={pr_number}, "
+                f"repo={resolved_repo}, remote={remote}): "
+                f"{_redact(str(exc))}"
+            ],
+            "pr_lookup": "ok",
+            "resolved_repo": resolved_repo,
+            "remote": remote,
+            "pr_number": pr_number,
+            "head_sha": (pr.get("head") or {}).get("sha"),
+        }
+    if not isinstance(comments, list):
+        comments = []
+    return {
+        "success": True,
+        "comments": list(comments[:limit]),
+        "reasons": [],
+        "pr_lookup": "ok",
+        "resolved_repo": resolved_repo,
+        "remote": remote,
+        "pr_number": pr_number,
+        "head_sha": (pr.get("head") or {}).get("sha"),
+    }
+
+
 def _list_pr_lease_comments(
     pr_number: int,
     *,
@@ -2660,16 +2739,15 @@ def _list_pr_lease_comments(
     limit: int = 100,
 ) -> list[dict]:
     """Fetch PR/issue thread comments used for reviewer/conflict-fix leases."""
-    h, o, r = _resolve(remote, host, org, repo)
-    auth = _auth(h)
-    api = f"{repo_api_url(h, o, r)}/issues/{pr_number}/comments"
-    comments = api_request("GET", api, auth)
-    # Fail safe to no lease comments when the API returns a non-list payload
-    # (e.g. an error object such as an HTTP 401 body): lease state can only be
-    # proven from real comment entries, never inferred from an error shape (#485).
-    if not isinstance(comments, list):
-        return []
-    return list(comments[:limit])
+    fetched = _fetch_pr_lease_comments_safe(
+        pr_number,
+        remote=remote,
+        host=host,
+        org=org,
+        repo=repo,
+        limit=limit,
+    )
+    return fetched["comments"]
 
 
 def _pr_work_lease_reviewer_block(
@@ -7052,22 +7130,39 @@ def gitea_assess_conflict_fix_push(
             "reasons": read_block,
             "permission_report": _permission_block_report("gitea.read"),
         }
-    comments = _list_pr_lease_comments(
+    fetched = _fetch_pr_lease_comments_safe(
         pr_number,
         remote=remote,
         host=host,
         org=org,
         repo=repo,
+        require_open=True,
     )
-    return pr_work_lease.assess_conflict_fix_push(
+    if not fetched["success"]:
+        return {
+            "push_allowed": False,
+            "block": True,
+            "reasons": fetched["reasons"],
+            "pr_lookup": fetched.get("pr_lookup"),
+            "resolved_repo": fetched.get("resolved_repo"),
+            "remote": fetched.get("remote"),
+            "pr_number": pr_number,
+            "assessment_failed": True,
+        }
+    assessment = pr_work_lease.assess_conflict_fix_push(
         pr_number=pr_number,
-        comments=comments,
+        comments=fetched["comments"],
         branch_head_before=branch_head_before,
         branch_head_after=branch_head_after,
         worktree_path=worktree_path,
         push_cwd=push_cwd,
         is_fast_forward=is_fast_forward,
     )
+    assessment["pr_lookup"] = fetched.get("pr_lookup")
+    assessment["resolved_repo"] = fetched.get("resolved_repo")
+    assessment["remote"] = fetched.get("remote")
+    assessment["live_pr_head_sha"] = fetched.get("head_sha")
+    return assessment
 
 
 @mcp.tool()