feat: remove core.telemetry as a dependency of llama_stack.apis (#4064)

# What does this PR do?

Remove circular dependency by moving tracing from API protocol
definitions
 to router implementation layer.

This gets us closer to having a self contained API package with no other
cross-cutting dependencies to other parts of the llama stack codebase.
To the best of our ability, the llama_stack.api should only be type and
protocol definitions.

  Changes:
- Create apis/common/tracing.py with marker decorator (zero core
dependencies)
- Add the _new_ `@telemetry_traceable` marker decorator to 11 protocol
classes
- Apply actual tracing in core/resolver.py in `instantiate_provider`
based on protocol marker
- Move MetricResponseMixin from core to apis (it's an API response type)
  - APIs package is now self-contained with zero core dependencies

The tracing functionality remains identical - actual trace_protocol from
core
is applied to router implementations at runtime when both telemetry is
enabled
  and the protocol has the `__marked_for_tracing__` marker.

  ## Test Plan

  Manual integration test confirms identical behavior to main branch:

  ```bash
  llama stack list-deps --format uv starter | sh
  export OLLAMA_URL=http://localhost:11434
  llama stack run starter

  curl -X POST http://localhost:8321/v1/chat/completions \
    -H "Content-Type: application/json" \
    -d '{"model": "ollama/gpt-oss:20b",
         "messages": [{"role": "user", "content": "Say hello"}],
         "max_tokens": 10}'
         
```

  Verified identical between main and this branch:
  - trace_id present in response
  - metrics array with prompt_tokens, completion_tokens, total_tokens
  - Server logs show trace_protocol applied to all routers

  Existing telemetry integration tests (tests/integration/telemetry/) validate
  trace context propagation and span attributes.


relates to #3895

---------

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

src/llama_stack/apis/conversations/conversations.py

@@ -20,8 +20,8 @@ from llama_stack.apis.agents.openai_responses import (
     OpenAIResponseOutputMessageMCPListTools,
     OpenAIResponseOutputMessageWebSearchToolCall,
 )
+from llama_stack.apis.common.tracing import telemetry_traceable
 from llama_stack.apis.version import LLAMA_STACK_API_V1
-from llama_stack.core.telemetry.trace_protocol import trace_protocol
 from llama_stack.schema_utils import json_schema_type, register_schema, webmethod
 
 Metadata = dict[str, str]
@@ -157,7 +157,7 @@ class ConversationItemDeletedResource(BaseModel):
 
 
 @runtime_checkable
-@trace_protocol
+@telemetry_traceable
 class Conversations(Protocol):
     """Conversations