Feat/prompt caching

[Unshure]

Summary

• Add CacheConfig with strategy="auto" for automatic prompt caching in BedrockModel
• Cache points are injected at the end of the last assistant message before each model call
• Supports all Claude models on Bedrock that have prompt caching capability

Usage

from strands import Agent
from strands.models import BedrockModel, CacheConfig

model = BedrockModel(
    model_id="us.anthropic.claude-sonnet-4-5-20250929-v1:0",
    cache_config=CacheConfig(strategy="auto")
)
agent = Agent(model=model)

Test plan

• Unit tests for cache point injection logic
• Integration test with Claude models on Bedrock confirming cache hits

Closes strands-agents#1432

[Unshure]

/strands review

[github-actions]

Issue: Using logger.warning here seems too aggressive - this is expected behavior when cache_config manages cache points automatically.

Suggestion: Consider using logger.debug instead since stripping existing cache points is the intended behavior in auto mode, not a warning condition.

[github-actions]

Issue: The PR description mentions "Integration test with Claude models on Bedrock confirming cache hits" but I couldn't find any integration test in tests_integ/ for the new cache_config feature.

Suggestion: If the integration test exists, please ensure it's included in this PR. If it was tested manually, consider documenting how to verify cache hits in the PR description.

[github-actions]

Review Summary

Assessment: Comment (Minor changes requested)

Key Themes:

• Clean, well-structured implementation of automatic prompt caching
• Good test coverage for the new functionality
• Minor issues: unrelated file modification and logging level choice

The implementation correctly injects cache points into cleaned_messages (not mutating original messages), handles non-Claude models gracefully, and properly strips existing cache points when in auto mode.

Minor Items:

1. tests/strands/agent/test_agent.py has unrelated changes (extra blank lines)
2. Consider logger.debug instead of logger.warning for stripped cache points (expected behavior in auto mode)
3. Clarify the integration test mentioned in the PR description

Nice work on this feature! 🎉