fix(robomp): thread commit_id/head_sha/patch through proxy backend

Follow-up fix round for 71d608aae6 (validate PR review comment
anchors against the diff before submitting). The validation path
broke in several places the original commit did not cover:

- GitHubBackend.submit_review gained no commit_id parameter, so
  submitting through GitHubProxyClient raised a production
  TypeError on a path the validation code depended on.
- PullRequestInfo lacked head_sha, so the Forgejo commit_id
  fallback raised AttributeError; it is now parsed in
  _pr_from_payload and carried through the proxy round-trip.
- _pr_file_from dropped the file patch, silently no-oping anchor
  validation for anything routed through the proxy; the patch is
  now forwarded.
- The hunk parser treated any +++/--- line as a file header,
  desyncing line counters when added/removed content began with
  those prefixes; file headers are now recognized only before
  the first hunk.
- Reworded the comment to "Forgejo only" to match the actual
  backend behavior.

Adds tests for forgejo commit_id fetch, fallback double-failure,
empty-patch fail-open, LEFT-side anchoring, proxy commit_id
validation, and file-creation hunk boundaries.

python/robomp/src/proxy_client.py

@@ -334,17 +334,21 @@ class GitHubProxyClient:
         body: str,
         event: str,
         comments: list[Mapping[str, Any]],
+        commit_id: str | None = None,
     ) -> PullRequestReviewInfo:
+        json_body: dict[str, Any] = {
+            "repo": repo,
+            "pr_number": pr_number,
+            "body": body,
+            "event": event,
+            "comments": comments,
+        }
+        if commit_id:
+            json_body["commit_id"] = commit_id
         data = await self._request(
             "POST",
             "/gh/v1/submit_pr_review",
-            json_body={
-                "repo": repo,
-                "pr_number": pr_number,
-                "body": body,
-                "event": event,
-                "comments": comments,
-            },
+            json_body=json_body,
         )
         return _pr_review_from(data)
 
@@ -602,6 +606,7 @@ def _pr_file_from(data: Any) -> PullRequestFileInfo:
         status=str(data.get("status") or ""),
         additions=int(data.get("additions") or 0),
         deletions=int(data.get("deletions") or 0),
+        patch=str(data.get("patch") or ""),
     )
 
 
@@ -619,6 +624,7 @@ def _pr_from(data: Any) -> PullRequestInfo:
         head_repo=str(data.get("head_repo") or ""),
         title=str(data.get("title") or ""),
         body=str(data.get("body") or ""),
+        head_sha=str(data.get("head_sha") or ""),
     )