fix(xai): OAuth Responses native web_search, incomplete guard, grok-composer context

- model_metadata: grok-composer-2.5-fast → 262144 (OAuth slug not in /v1/models)
- codex transport: inject native {"type":"web_search"} for is_xai_responses;
  drop client web_search to avoid duplicate-name 400s
- codex adapter: do not treat in-progress server-side *_call items as incomplete
- tests: adapter, transport build_kwargs, model_metadata, oauth recovery

tests/agent/test_model_metadata.py

@@ -142,6 +142,7 @@ class TestDefaultContextLengths:
                 ("grok-4", 256000),
                 ("grok-4-0709", 256000),
                 ("grok-build-0.1", 256000),
+                ("grok-composer-2.5-fast", 262144),
                 ("grok-code-fast-1", 256000),
                 ("grok-3", 131072),
                 ("grok-3-mini", 131072),