fix(xai): scope native web_search to swap-only + reconcile composer ctx to 200k

Salvage corrections on top of @XVVH's #44341:
- Make native web_search injection a 1:1 swap for an already-present client
  web_search function, NOT an additive grant. The original unconditionally
  appended {"type":"web_search"} on every is_xai_responses turn with any
  tools, force-enabling Grok server-side search even when the user never
  enabled the web toolset (bypassing Hermes web-provider config + tool-trace
  plumbing). Now gated on a client web_search actually being present.
- Reconcile grok-composer context to 200000 (merged in #47908) rather than
  262144; 200k is xAI's published usable context window for Composer 2.5,
  262144 is the /v1/responses input+output budget.
- Update tests to match scoped behavior + add a no-web-toolset guard test.
- AUTHOR_MAP entry for #44341 salvage.

Incomplete-guard (server-side *_call items at in_progress no longer flip
has_incomplete_items) and preflight built-in-tool allowlist kept as-is.

tests/agent/test_model_metadata.py

@@ -142,7 +142,7 @@ class TestDefaultContextLengths:
                 ("grok-4", 256000),
                 ("grok-4-0709", 256000),
                 ("grok-build-0.1", 256000),
-                ("grok-composer-2.5-fast", 262144),
+                ("grok-composer-2.5-fast", 200000),
                 ("grok-code-fast-1", 256000),
                 ("grok-3", 131072),
                 ("grok-3-mini", 131072),