refactor(transports/codex): trim duplicated cache-key comments

The xAI prompt_cache_key block carried two long comment paragraphs
that either restated setdefault semantics, narrated the SDK
type-validation mechanism, or recapped the historical motivation for
the extra_body indirection — all already covered by the test
docstring at test_xai_responses_sends_cache_key_via_extra_body
(which links to the xAI docs). Also restored the truncated link in
the body-injection comment.

No behavior change.

agent/transports/codex.py

@@ -102,11 +102,8 @@ class ResponsesApiTransport(ProviderTransport):
             kwargs["parallel_tool_calls"] = True
 
         session_id = params.get("session_id")
-        # xAI's Responses API uses `prompt_cache_key` (body-level) as the
-        # cache-routing key, not a top-level kwarg — the body-field
-        # injection below survives openai SDK builds whose
-        # Responses.stream() signature drops the kwarg. Everything else
-        # that ISN'T github/xAI keeps using the typed kwarg.
+        # xAI Responses takes prompt_cache_key in extra_body (set further
+        # down); GitHub Models opts out of cache-key routing entirely.
         if not is_github_responses and not is_xai_responses and session_id:
             kwargs["prompt_cache_key"] = session_id
 
@@ -172,15 +169,10 @@ class ResponsesApiTransport(ProviderTransport):
             merged_extra_headers["x-grok-conv-id"] = session_id
             kwargs["extra_headers"] = merged_extra_headers
 
-            # xAI Responses cache-routing field. Lives in the request body
-            # (per https://docs.x.ai/.../prompt-caching/maximizing-cache-hits),
-            # so we ship it via extra_body — the openai SDK serializes
-            # extra_body fields into the JSON body without per-field type
-            # validation, sidestepping the TypeError that fires on
-            # Responses.stream() builds whose `prompt_cache_key` kwarg has
-            # been dropped. Setdefault preserves a caller-supplied value
-            # (e.g. request_overrides.extra_body.prompt_cache_key) over
-            # the auto-derived session_id.
+            # xAI Responses cache-routing — body-level field per
+            # https://docs.x.ai/developers/advanced-api-usage/prompt-caching/maximizing-cache-hits.
+            # Sent via extra_body (not the typed kwarg) so it survives openai
+            # SDK builds whose Responses.stream() signature has dropped the field.
             existing_extra_body = kwargs.get("extra_body")
             merged_extra_body: Dict[str, Any] = {}
             if isinstance(existing_extra_body, dict):