[bugfix] no shield_call when there's no shields configured (#642)

# What does this PR do?

**Why**
- When AgentConfig has no `input_shields` / `output_shields` defined, we
still outputs a shield_call step with violation=None. This is impossible
to distinguish the case b/w (1) no violation from running shields v.s.
(2) no shields call

**What**
- We should not have a shield_call step when no `input_shields` /
`output_shields` are defined.

- Also removes a never reached try/catch code block in agent loop.
`run_multiple_shields` is never called in the try block (verified by
stacktrace print)

**Side Note**
- pre-commit fix

## Test Plan

Tested w/ DirectClient via:
https://gist.github.com/yanxi0830/b48f2a53b6f5391b9ff1e39992bc05b3

**No Shields**
<img width="858" alt="image"
src="https://github.com/user-attachments/assets/67319370-329f-4954-bd16-d21ce54c6ebf"
/>

**With Input + Output Shields**
<img width="854" alt="image"
src="https://github.com/user-attachments/assets/75ab1bee-3ba9-4549-ab51-23210be83da7"
/>

**Input Shields Only**
<img width="858" alt="image"
src="https://github.com/user-attachments/assets/1897206b-13dd-4ea5-92c2-b39bf68e9286"
/>


E2E pytest
```
LLAMA_STACK_BASE_URL=http://localhost:5000 pytest -v ./tests/client-sdk/agents/test_agents.py
```

## Sources

Please link relevant resources if necessary.


## Before submitting

- [ ] This PR fixes a typo or improves the docs (you can dismiss the
other checks if that's the case).
- [ ] Ran pre-commit to handle lint / formatting issues.
- [ ] 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

@@ -239,6 +239,7 @@ class ChatAgent(ShieldRunnerMixin):
         # return a "final value" for the `yield from` statement. we simulate that by yielding a
         # final boolean (to see whether an exception happened) and then explicitly testing for it.
 
+        if len(self.input_shields) > 0:
             async for res in self.run_multiple_shields_wrapper(
                 turn_id, input_messages, self.input_shields, "user-input"
             ):
@@ -262,6 +263,7 @@ class ChatAgent(ShieldRunnerMixin):
         # for output shields run on the full input and output combination
         messages = input_messages + [final_response]
 
+        if len(self.output_shields) > 0:
             async for res in self.run_multiple_shields_wrapper(
                 turn_id, messages, self.output_shields, "assistant-output"
             ):
@@ -531,7 +533,6 @@ class ChatAgent(ShieldRunnerMixin):
                     input_messages = input_messages + [message]
             else:
                 log.info(f"{str(message)}")
-                try:
                 tool_call = message.tool_calls[0]
 
                 name = tool_call.tool_name
@@ -597,39 +598,6 @@ class ChatAgent(ShieldRunnerMixin):
 
                 # TODO: add tool-input touchpoint and a "start" event for this step also
                 # but that needs a lot more refactoring of Tool code potentially
-                    yield AgentTurnResponseStreamChunk(
-                        event=AgentTurnResponseEvent(
-                            payload=AgentTurnResponseStepCompletePayload(
-                                step_type=StepType.shield_call.value,
-                                step_details=ShieldCallStep(
-                                    step_id=str(uuid.uuid4()),
-                                    turn_id=turn_id,
-                                    violation=None,
-                                ),
-                            )
-                        )
-                    )
-
-                except SafetyException as e:
-                    yield AgentTurnResponseStreamChunk(
-                        event=AgentTurnResponseEvent(
-                            payload=AgentTurnResponseStepCompletePayload(
-                                step_type=StepType.shield_call.value,
-                                step_details=ShieldCallStep(
-                                    step_id=str(uuid.uuid4()),
-                                    turn_id=turn_id,
-                                    violation=e.violation,
-                                ),
-                            )
-                        )
-                    )
-
-                    yield CompletionMessage(
-                        content=str(e),
-                        stop_reason=StopReason.end_of_turn,
-                    )
-                    yield False
-                    return
 
                 if out_attachment := interpret_content_as_attachment(
                     result_message.content

llama_stack/providers/remote/inference/bedrock/bedrock.py

@@ -7,6 +7,7 @@
 from typing import *  # noqa: F403
 import json
 import uuid
+
 from botocore.client import BaseClient
 from llama_models.datatypes import CoreModelId
 

llama_stack/templates/bedrock/bedrock.py

@@ -7,12 +7,14 @@
 from pathlib import Path
 
 from llama_models.sku_list import all_registered_models
+
+from llama_stack.apis.models import ModelInput
 from llama_stack.distribution.datatypes import Provider
 
 from llama_stack.providers.inline.memory.faiss.config import FaissImplConfig
-from llama_stack.templates.template import DistributionTemplate, RunConfigSettings
 from llama_stack.providers.remote.inference.bedrock.bedrock import MODEL_ALIASES
-from llama_stack.apis.models import ModelInput
+from llama_stack.templates.template import DistributionTemplate, RunConfigSettings
+
 
 def get_distribution_template() -> DistributionTemplate:
     providers = {