fix(model-routes): harden _probe_endpoint against malformed model-list responses (#4789)

* fix(model-routes): harden _probe_endpoint against malformed model-list responses

_probe_endpoint parsed model lists with data.get(...) at four sites without
checking that data is a dict, and built the list with a truthiness-only
filter. A /models (or /api/tags) endpoint returning HTTP 200 with valid but
non-dict JSON ([], "x", null, 123) made data.get(...) raise AttributeError,
and a non-string id like 123 passed the filter and then hit .startswith() /
.lower() in the Z.AI/Kimi curated merge and _is_chat_model(). Both errors are
swallowed by the broad except Exception, but the comprehension dies mid-list
so the ENTIRE probed model list is discarded and the endpoint silently
degrades — masking a misconfigured/non-compliant upstream as "no models".

- Guard each data.get(...) with isinstance(data, dict) so a non-dict body
  falls through the existing `or []` default.
- Restrict the OpenAI and Ollama model-list comprehensions to non-empty str
  values, protecting the .startswith() merges and both _is_chat_model calls.
- Add an isinstance guard at the top of _is_chat_model (defense in depth for
  all four call sites).

No behavior change for well-formed {"data":[...]} / {"models":[...]}
responses. Adds regression tests (non-dict body via caplog, mixed/all
non-string ids, _is_chat_model boundary) that fail before the fix and pass
after.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* refactor(model-routes): extract _openai_model_ids / _ollama_model_names helpers

Per review on #4789: the malformed-response guards were inlined four times in
_probe_endpoint (two OpenAI-id comprehensions, two Ollama-name comprehensions).
Pull each into a small, directly-testable helper so the security-relevant
parsing lives in one place and a future malformed-shape fix doesn't have to be
applied in four spots (CONTRIBUTING flags repeated logic for this reason).

Behavior is unchanged. Adds direct unit tests for both helpers (non-dict body,
non-string ids, non-dict entries, name>model precedence).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_model_routes.py

@@ -403,6 +403,12 @@ class TestIsChatModel:
     def test_legacy_openai_instruct_is_not_chat(self):
         assert _is_chat_model("gpt-3.5-turbo-instruct") is False
 
+    @pytest.mark.parametrize("bad", [None, 123, 4.5, ["x"], {"a": 1}])
+    def test_non_string_id_is_treated_as_chat(self, bad):
+        # Defensive boundary: a non-compliant upstream can yield a non-string
+        # model id; it must not crash on .lower() (treated as chat-capable).
+        assert _is_chat_model(bad) is True
+
 
 # ── _classify_endpoint ──