feat(types): add native audio content block support

[strands-agent]

Description

Add native audio content block support to the SDK's type system, following the established pattern for video, image, and document content.

Changes

1. Types (src/strands/types/media.py):

   • Add AudioFormat literal type (mp3, wav, flac, ogg, webm)
   • Add AudioSource TypedDict with bytes attribute
   • Add AudioContent TypedDict with format and source attributes

2. ContentBlock (src/strands/types/content.py):

   • Add audio: AudioContent field to the ContentBlock TypedDict

3. Bedrock Provider (src/strands/models/bedrock.py):

   • Add audio handling in _format_request_message_content() following the video pattern

4. LlamaCpp Provider (src/strands/models/llamacpp.py):

   • Update to use native AudioContent types instead of cast(Dict[str, Any], content)

5. Tests (tests/strands/models/test_bedrock.py):

   • Add test_format_request_filters_audio_content_blocks test

Benefits

• Type Safety: No more cast(Dict[str, Any], content) workarounds
• Consistency: Audio follows the same pattern as video, image, and document
• Model Support: Enables type-safe audio handling for Bedrock (Nova Sonic), LlamaCpp (Qwen2.5-Omni), and future providers

Usage Example

from strands.types.content import ContentBlock
from strands.types.media import AudioContent

audio_block: ContentBlock = {
    "audio": {
        "format": "mp3",
        "source": {"bytes": audio_bytes}
    }
}

response = agent([audio_block, {"text": "What do you hear in this audio?"}])

Related Issues

Closes #866

Documentation PR

No documentation changes required - this adds types that follow existing patterns.

Type of Change

New feature

Testing

• I ran hatch run prepare (formatter + linter + mypy + tests)
• Unit test passes: test_format_request_filters_audio_content_blocks
• All existing tests continue to pass

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