[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/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 = {