fix(open-swe): surface reviewer diff-prep failures instead of swallowing (#1260)

When `_ensure_repo_checked_out` or `compute_diff_in_sandbox` fail, the
reviewer used to log+continue and hand the agent an empty diff. The
agent would then emit a misleading "no issues found" review and the
underlying error stayed buried in server logs.

Now the helpers raise on non-zero exit codes (with the sandbox output)
and the prep block re-raises, so the failure shows up in the LangSmith
run trace and we can debug the real issue.

agent/reviewer.py

@@ -179,7 +179,14 @@ async def _ensure_repo_checked_out(
     )
     import asyncio
 
-    await asyncio.to_thread(sandbox_backend.execute, script)
+    result = await asyncio.to_thread(sandbox_backend.execute, script)
+    exit_code = getattr(result, "exit_code", None)
+    if exit_code not in (0, None):
+        output = getattr(result, "output", "") or ""
+        raise RuntimeError(
+            f"Repo checkout failed (exit {exit_code}) for {owner}/{repo} "
+            f"@ {head_sha} (base {base_sha}). Script output:\n{output}"
+        )
 
 
 def _build_first_review_context(
@@ -326,7 +333,11 @@ async def get_reviewer_agent(config: RunnableConfig) -> Pregel:
                 )
             diff_line_set = compute_diff_line_set(diff_text)
         except Exception:
+            # Don't swallow: an empty diff makes the agent emit a misleading
+            # "no issues found" review. Re-raise so the failure surfaces in
+            # the LangSmith trace with stderr from the sandbox commands.
             logger.exception("Reviewer prep failed for thread %s", thread_id)
+            raise
 
     config["configurable"]["diff_text"] = diff_text
     config["configurable"]["diff_line_set"] = {

agent/reviewer_diff.py

@@ -208,6 +208,13 @@ async def compute_diff_in_sandbox(
     operator = "..." if merge_base else ".."
     cmd = f"cd {work_dir} && git diff --no-color {base_ref}{operator}{head_ref}"
     result = await asyncio.to_thread(sandbox_backend.execute, cmd)
+    exit_code = getattr(result, "exit_code", None)
+    if exit_code not in (0, None):
+        output = _stdout_from_result(result)
+        raise RuntimeError(
+            f"git diff failed (exit {exit_code}) for "
+            f"{base_ref}{operator}{head_ref} in {work_dir}. Output:\n{output}"
+        )
     return _stdout_from_result(result)