fix(robomp): validate PR review comment anchors against the diff before submitting

A single comment on a line outside a diff hunk 422s the whole review on
GitHub, which the 422/500 fallback (ported here from the Forgejo work,
including payload shaping and commit_id) then degraded into plain issue
comments. Now we fetch /pulls/{n}/files, compute per-hunk anchorable line
sets, drop non-anchorable comments, fold them into the review summary, and
only fall back to issue comments for genuine 422/500s that survive
filtering. Files-fetch failure fails open (submit unfiltered).

python/robomp/src/github_client.py

@@ -77,6 +77,7 @@ class PullRequestFileInfo:
     status: str
     additions: int
     deletions: int
+    patch: str = ""
 
 
 @dataclass(slots=True, frozen=True)
@@ -179,7 +180,13 @@ def _parse_retry_after(resp: httpx.Response) -> float | None:
 class GitHubClient:
     """Async + sync facades over a small slice of the GitHub REST API."""
 
-    def __init__(self, token: str, *, transport: httpx.BaseTransport | None = None) -> None:
+    def __init__(
+        self,
+        token: str,
+        *,
+        transport: httpx.BaseTransport | None = None,
+        platform: str = "github",
+    ) -> None:
         self._token = token
         self._headers = {
             "Authorization": f"Bearer {token}",
@@ -188,6 +195,7 @@ class GitHubClient:
             "User-Agent": "robomp/0.1",
         }
         self._transport = transport
+        self._platform = platform
 
     def _client(self) -> httpx.Client:
         return httpx.Client(
@@ -581,6 +589,26 @@ class GitHubClient:
             f"/repos/{repo}/issues/{number}/labels/{encoded}",
         )
 
+    def _review_comments_payload(self, comments: list[Mapping[str, Any]]) -> list[dict[str, Any]]:
+        """Adapt canonical host-tool comment shape to the wire schema for this platform.
+
+        GitHub keeps line/side/start_line/start_side; Forgejo/Gitea only reads
+        path/body/new_position (+old_position), so github-only keys are dropped
+        and `line` is mapped to `new_position` for RIGHT-side comments or
+        `old_position` for LEFT-side (removed-line) comments.
+        """
+        if self._platform != "forgejo":
+            return [dict(c) for c in comments]
+        payload: list[dict[str, Any]] = []
+        for c in comments:
+            entry: dict[str, Any] = {"path": c["path"], "body": c["body"]}
+            if str(c.get("side", "RIGHT")).upper() == "LEFT":
+                entry["old_position"] = c["line"]
+            else:
+                entry["new_position"] = c["line"]
+            payload.append(entry)
+        return payload
+
     async def submit_pr_review(
         self,
         *,
@@ -589,12 +617,12 @@ class GitHubClient:
         body: str,
         event: str,
         comments: list[Mapping[str, Any]],
+        commit_id: str | None = None,
     ) -> PullRequestReviewInfo:
-        data = await self.request(
-            "POST",
-            f"/repos/{repo}/pulls/{pr_number}/reviews",
-            json={"body": body, "event": event, "comments": comments},
-        )
+        payload: dict[str, Any] = {"body": body, "event": event, "comments": self._review_comments_payload(comments)}
+        if commit_id:
+            payload["commit_id"] = commit_id
+        data = await self.request("POST", f"/repos/{repo}/pulls/{pr_number}/reviews", json=payload)
         return _pr_review_from_payload(data)
 
     async def add_assignees(self, repo: str, number: int, assignees: list[str]) -> None:
@@ -750,6 +778,7 @@ def _pr_file_from_payload(data: Mapping[str, 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 ""),
     )