fix(agent/tools): surface github search error responses (#17064)

### Summary

`agent/tools/github.py` indexes `response["items"]` directly after
`requests.get(...).json()`. the github search api returns `{"message":
...}` **without** an `items` key on realistic conditions — a rate limit
(403/429; this tool sends no auth token, so the unauthenticated ~10
req/min search limit is easy to hit) or an invalid query (422). that
raised `KeyError('items')`, which the tool's retry loop then surfaced to
the model as the opaque `"GitHub error: 'items'"` instead of the real
reason.

this guards the missing key and raises the api's actual `message` (with
a clear fallback) into the existing retry/`_ERROR` path, so the model
sees e.g. `"GitHub error: API rate limit exceeded ..."`. valid responses
are unchanged.

adds `test/unit_test/agent/tools/test_github_unit.py` covering the
rate-limit response (asserts the real message is surfaced, no
`KeyError`) and the normal result path.

Co-authored-by: Yaroslav98214 <diakovichyaroslav30@gmail.com>
Co-authored-by: Haruko386 <tryeverypossible@163.com>

agent/tools/github.py

@@ -76,8 +76,15 @@ class GitHub(ToolBase, ABC):
                 if self.check_if_canceled("GitHub processing"):
                     return
 
-                self._retrieve_chunks(response["items"], get_title=lambda r: r["name"], get_url=lambda r: r["html_url"], get_content=lambda r: str(r["description"]) + "\n stars:" + str(r["watchers"]))
-                self.set_output("json", response["items"])
+                # the github search api reports rate limits (403/429) and invalid
+                # queries (422) through a "message" field and omits "items"; surface
+                # that instead of raising a cryptic KeyError on the missing key.
+                if "items" not in response:
+                    raise Exception(response.get("message", "GitHub search returned no items."))
+
+                items = response["items"]
+                self._retrieve_chunks(items, get_title=lambda r: r["name"], get_url=lambda r: r["html_url"], get_content=lambda r: str(r["description"]) + "\n stars:" + str(r["watchers"]))
+                self.set_output("json", items)
                 return self.output("formalized_content")
             except Exception as e:
                 if self.check_if_canceled("GitHub processing"):