feat: add cache_messages to bedrock model

[DayuanJiang]

Description

Introduced the cache_messages configuration parameter to BedrockModel to enable message caching for intermediate steps.

When cache_messages = "default", the agent will add a cache point at the end of the message list each time it calls a Bedrock model. This allows the agent to cache intermediate steps effectively while preserving any existing cache points users may have set.

Key implementation details:

• Adds a new cache point at the end of the last message
• Creates copies of message structures to avoid modifying the original messages passed to format_request

Related Issues

#404 #1015

Documentation PR

None

Type of Change

New feature

Testing

• test_format_request_cache_messages(): Validates that existing cache points are preserved and a new cache point is added at the end

• test_format_request_cache_messages_does_not_modify_original(): Verifies that the original messages passed to format_request are not mutated

  • Tests preservation of existing cache points from multiple messages
  • Verifies correct addition of new cache point at the end of the last message
  • Confirms complete request structure matches expectations
  • Ensures no mutation of original message objects

• 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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[pgrayy]

Can you explain the purpose of clearing cachePoints from previously processed messages?

[DayuanJiang]

Thanks for pointing that out. Originally, the implementation would add a cache point to the last message on each call. To avoid accumulating too many cache points across multiple calls, I decided to remove all existing cache points before adding the new one.

However, I realized that this approach was problematic because it modified the original messages passed to the function. So I modified the code to create a copy of the messages and add the cache point to the copied version, which means clearing existing cachePoints is no longer necessary.

I have also updated the PR description to reflect this change.