[#407] Agents: Avoid calling tools that haven't been explicitly enabled (#637)

# What does this PR do?

Contributes to issue (#407)

tl;dr - @subramen was getting a 500 error because llama-stack called
code_interpreter when it never was defined as a tool.

Prevents failures like:

<img width="544" alt="image"
src="https://github.com/user-attachments/assets/392683d2-4670-414c-aaba-07ebc006d748"
/>

```
# Server side
Traceback (most recent call last):
  File "/opt/conda/envs/llamastack-vllm-stack/lib/python3.10/site-packages/llama_stack/distribution/server/server.py", line 206, in sse_generator
    async for item in await event_gen:
  File "/opt/conda/envs/llamastack-vllm-stack/lib/python3.10/site-packages/llama_stack/providers/impls/meta_reference/agents/agents.py", line 138, in _create_agent_turn_streaming
    async for event in agent.create_and_execute_turn(request):
  File "/opt/conda/envs/llamastack-vllm-stack/lib/python3.10/site-packages/llama_stack/providers/impls/meta_reference/agents/agent_instance.py", line 179, in create_and_execute_turn
    async for chunk in self.run(
  File "/opt/conda/envs/llamastack-vllm-stack/lib/python3.10/site-packages/llama_stack/providers/impls/meta_reference/agents/agent_instance.py", line 252, in run
    async for res in self._run(
  File "/opt/conda/envs/llamastack-vllm-stack/lib/python3.10/site-packages/llama_stack/providers/impls/meta_reference/agents/agent_instance.py", line 560, in _run
    result_messages = await execute_tool_call_maybe(
  File "/opt/conda/envs/llamastack-vllm-stack/lib/python3.10/site-packages/llama_stack/providers/impls/meta_reference/agents/agent_instance.py", line 824, in execute_tool_call_maybe
    assert name in tools_dict, f"Tool {name} not found"
AssertionError: Tool code_interpreter not found
```

Instead, if the model hallucinates, we just let it hallucinate and let
the client know.

<img width="544" alt="image"
src="https://github.com/user-attachments/assets/d2418583-d45a-48db-b476-45a584f2986f"
/>

## Test Plan

<details>
<summary>pytest llama_stack/providers/tests/agents/test_agents.py -k
ollama</summary>

```
llama stack build --template ollama --image-type conda 
conda activate llamastack-ollama
```

```
llama_stack/providers/tests/agents/test_agents.py ..Fss                                                                                          [100%]

======================================================================= FAILURES =======================================================================
_________________________________________ TestAgents.test_rag_agent_as_attachments[--ollama][ollama] __________________________________________
llama_stack/providers/tests/agents/test_agents.py:261: in test_rag_agent_as_attachments
    turn_response = [
llama_stack/providers/tests/agents/test_agents.py:261: in <listcomp>
    turn_response = [
llama_stack/providers/inline/agents/meta_reference/agents.py:153: in _create_agent_turn_streaming
    async for event in agent.create_and_execute_turn(request):
llama_stack/providers/inline/agents/meta_reference/agent_instance.py:179: in create_and_execute_turn
    async for chunk in self.run(
llama_stack/providers/inline/agents/meta_reference/agent_instance.py:250: in run
    async for res in self._run(
llama_stack/providers/inline/agents/meta_reference/agent_instance.py:363: in _run
    rag_context, bank_ids = await self._retrieve_context(
llama_stack/providers/inline/agents/meta_reference/agent_instance.py:698: in _retrieve_context
    bank_id = await self._ensure_memory_bank(session_id)
llama_stack/providers/inline/agents/meta_reference/agent_instance.py:653: in _ensure_memory_bank
    await self.memory_banks_api.register_memory_bank(
llama_stack/providers/utils/telemetry/trace_protocol.py:101: in async_wrapper
    result = await method(self, *args, **kwargs)
llama_stack/distribution/routers/routing_tables.py:312: in register_memory_bank
    raise ValueError(
E   ValueError: Embeddings are now served via Inference providers. Please upgrade your run.yaml to include inline::sentence-transformer as an additional inference provider. See https://github.com/meta-llama/llama-stack/blob/main/llama_stack/templates/together/run.yaml for an example.
=============================================================== short test summary info ================================================================
FAILED llama_stack/providers/tests/agents/test_agents.py::TestAgents::test_rag_agent_as_attachments[--ollama] - ValueError: Embeddings are now served via Inference providers. Please upgrade your run.yaml to include inline::sentence-transformer as an additiona...
========================================== 1 failed, 2 passed, 2 skipped, 20 deselected, 5 warnings in 14.24s ==========================================
```

Unrelated test is failing (also failing on main)
</details>

<details>
<summary>Manual</summary>

Using this client code:
7ebc257b27/client.py

<img width="544" alt="Screenshot 2024-12-16 at 17 41 31"
src="https://github.com/user-attachments/assets/7425deaf-c94a-4dda-a635-922728e373f1"
/>

</details>

## Before submitting

- [ ] This PR fixes a typo or improves the docs (you can dismiss the
other checks if that's the case).
- [x] Ran pre-commit to handle lint / formatting issues.
- [x] Read the [contributor
guideline](https://github.com/meta-llama/llama-stack/blob/main/CONTRIBUTING.md),
      Pull Request section?
- [ ] Updated relevant documentation.
- [ ] Wrote necessary unit or integration tests.

llama_stack/providers/inline/agents/meta_reference/agent_instance.py

@@ -584,7 +584,7 @@ class ChatAgent(ShieldRunnerMixin):
                 tool_call = message.tool_calls[0]
 
                 name = tool_call.tool_name
-                if not isinstance(name, BuiltinTool):
+                if not isinstance(name, BuiltinTool) or name not in enabled_tools:
                     yield message
                     return