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/agent/test_prompt_caching.py

@@ -13,11 +13,18 @@ MARKER = {"type": "ephemeral"}
 
 
 class TestApplyCacheMarker:
-    def test_tool_message_gets_top_level_marker(self):
+    def test_tool_message_gets_top_level_marker_on_native_anthropic(self):
+        """Native Anthropic path: cache_control injected top-level (adapter moves it inside tool_result)."""
         msg = {"role": "tool", "content": "result"}
-        _apply_cache_marker(msg, MARKER)
+        _apply_cache_marker(msg, MARKER, native_anthropic=True)
         assert msg["cache_control"] == MARKER
 
+    def test_tool_message_skips_marker_on_openrouter(self):
+        """OpenRouter path: top-level cache_control on role:tool is invalid and causes silent hang."""
+        msg = {"role": "tool", "content": "result"}
+        _apply_cache_marker(msg, MARKER, native_anthropic=False)
+        assert "cache_control" not in msg
+
     def test_none_content_gets_top_level_marker(self):
         msg = {"role": "assistant", "content": None}
         _apply_cache_marker(msg, MARKER)

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]