fix(agent/tools): surface serpapi error responses in google search (#17062)

### Summary

`agent/tools/google.py` indexes `search["organic_results"]` directly
after `GoogleSearch(params).get_dict()`. serpapi returns `{"error":
...}` **without** an `organic_results` key on realistic conditions — an
invalid API key, an exhausted plan/quota, or a query that matched
nothing. that raised `KeyError('organic_results')`, which the tool's
retry loop then surfaced to the model as the opaque `"Google error:
'organic_results'"` instead of the real reason.

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

adds `test/unit_test/agent/tools/test_google_unit.py` covering the
error-response path (asserts the real message is surfaced, no
`KeyError`) and the normal result path, mirroring the existing
`test_googlescholar.py` pattern (skips when the `serpapi` SDK is
absent).

---------

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

agent/tools/google.py

@@ -498,13 +498,23 @@ class Google(ToolBase, ABC):
                 if self.check_if_canceled("Google processing"):
                     return
 
+                # serpapi reports an invalid key, exhausted quota or an empty result
+                # set through an "error" field and omits "organic_results"; surface that
+                # message instead of raising a cryptic KeyError on the missing key.
+                if "organic_results" not in search:
+                    raise RuntimeError(search.get("error", "SerpApi returned no organic_results."))
+
+                organic_results = search["organic_results"]
+                # a result may omit any of these; note the fallback of the "description"
+                # lookup is evaluated eagerly, so it has to be a .get() too or a result
+                # carrying a description but no snippet raises KeyError.
                 self._retrieve_chunks(
-                    search["organic_results"],
-                    get_title=lambda r: r["title"],
-                    get_url=lambda r: r["link"],
-                    get_content=lambda r: r.get("about_this_result", {}).get("source", {}).get("description", r["snippet"]),
+                    organic_results,
+                    get_title=lambda r: r.get("title", ""),
+                    get_url=lambda r: r.get("link", ""),
+                    get_content=lambda r: r.get("about_this_result", {}).get("source", {}).get("description", r.get("snippet", "")),
                 )
-                self.set_output("json", search["organic_results"])
+                self.set_output("json", organic_results)
                 return self.output("formalized_content")
             except Exception as e:
                 if self.check_if_canceled("Google processing"):