fix(prompt-caching): skip top-level cache_control on role:tool for OpenRouter

On the native Anthropic Messages API path, convert_messages_to_anthropic()
moves top-level cache_control on role:tool messages inside the tool_result
block. On OpenRouter (chat_completions), no such conversion happens — the
unexpected top-level field causes a silent hang on the second tool call.

Add native_anthropic parameter to _apply_cache_marker() and
apply_anthropic_cache_control(). When False (OpenRouter), role:tool messages
are skipped entirely. When True (native Anthropic), existing behaviour is
preserved.

Fixes #2362

tests/test_anthropic_adapter.py

@@ -718,7 +718,7 @@ class TestConvertMessages:
                 ],
             },
             {"role": "tool", "tool_call_id": "tc_1", "content": "result"},
-        ])
+        ], native_anthropic=True)
 
         _, result = convert_messages_to_anthropic(messages)
         user_msg = [m for m in result if m["role"] == "user"][0]