fix(bedrock): place cache point before non-PDF document blocks

[kevmyung]

Description

Closes #1966

PR #1821 moved cache point injection from the last assistant message to the last user message. This causes a ValidationException when the last user message contains non-PDF document blocks (md, csv, txt, html), because Bedrock's format conversion fails to preserve the type field on a cachePoint block placed directly after a non-PDF document block.

In _inject_cache_point, instead of unconditionally appending cachePoint to the end of the content array, scan for the first non-PDF document block and insert cachePoint before it. If no non-PDF document blocks exist, the existing behavior (append at end) is preserved.

Cache effectiveness is not impacted — Anthropic's prompt caching is prefix-based, so placing cachePoint before the document block still includes the document content in the cached prefix.

Related Issues

#1966
#1821

Documentation PR

N/A

Type of Change

Bug fix

Testing

• Added test: cache point inserted before non-PDF document block
• Added test: cache point appended at end with PDF-only document
• Added test: mixed PDF and non-PDF documents
• All existing cache point tests pass
• 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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