feat(bedrock): add guardrail_latest_message option

[aiancheruk]

Description

This PR adds a new guardrail_latest_message parameter to the BedrockModel that allows users to send only the last user message to AWS Bedrock Guardrails for evaluation, instead of the entire conversation history.

Key changes:

• Added guardrail_latest_message: bool = False parameter to BedrockConfig
• Modified _format_bedrock_messages() adding latest user message to guardContent when guardrail_latest_message=True
• Added comprehensive unit tests for the helper method and request formatting
• Added integration tests to verify the feature works end-to-end with AWS Bedrock Guardrails

Benefits:

• Reduces token usage and costs when using guardrails
• Enables conversation recovery after guardrail interventions (blocked content in history won't block future messages)

Follow up:
Use coding agent to add image in GuardContent

Related Issues

#999

Documentation PR

strands-agents/docs#340

Type of Change

New feature

Testing

How have you tested the change? Verify that the changes do not break functionality or introduce warnings in consuming repositories: agents-docs, agents-tools, agents-cli

• I ran hatch run prepare

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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[aiancheruk]

Related: #999

[JackYPCOnline]

@aiancheruk Thank you for raising this PR! Could you refine the helper function a bit? Once that's done, I'll be happy to merge it.
Update: I'll update the PR based on your code to expedite the merge.

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/strands/models/bedrock.py	87.50%	0 Missing and 1 partial ⚠️

📢 Thoughts on this report? Let us know!

he was out, this PRs has been reviewed by Nick

[aiancheruk]

Thanks for your help @JackYPCOnline @Unshure @pgrayy!