refactor: rename trace_protocol marker to mark_as_traced

Rename the marker decorator in apis/common/tracing.py from trace_protocol
to mark_as_traced to disambiguate it from the actual tracing implementation
decorator in core/telemetry/trace_protocol.py.

Changes:
- Rename decorator: trace_protocol -> mark_as_traced
- Rename attribute: __trace_protocol__ -> __marked_for_tracing__
- Update all API protocol files to use new decorator name
- Update router logic to check for new attribute name

This makes it clear that the marker decorator is metadata-only and doesn't
perform actual tracing, while the core decorator does the implementation.

Signed-off-by: Charlie Doern <cdoern@redhat.com>

src/llama_stack/apis/conversations/conversations.py

@@ -20,7 +20,7 @@ from llama_stack.apis.agents.openai_responses import (
     OpenAIResponseOutputMessageMCPListTools,
     OpenAIResponseOutputMessageWebSearchToolCall,
 )
-from llama_stack.apis.common.tracing import trace_protocol
+from llama_stack.apis.common.tracing import mark_as_traced
 from llama_stack.apis.version import LLAMA_STACK_API_V1
 from llama_stack.schema_utils import json_schema_type, register_schema, webmethod
 
@@ -183,7 +183,7 @@ class ConversationItemDeletedResource(BaseModel):
 
 
 @runtime_checkable
-@trace_protocol
+@mark_as_traced
 class Conversations(Protocol):
     """Conversations