[FEATURE]: Bedrock prompt caching via cache_point_ttl provider option

[bainos]

• I have verified this feature I'm about to request hasn't been suggested before.
  Bedrock prompt caching works but there is no way to configure the cache TTL from opencode. This adds a cache_point_ttl option to the Bedrock provider in opencode.json, accepting 5m or 1h. Tested on Haiku 4.5, Sonnet 4.5/4.6, Opus 4.5/4.6.

[github-actions]

This issue doesn't fully meet our contributing guidelines.

What needs to be fixed:

• The feature request template requires checking the box verifying that this feature hasn't been suggested before (required field). Please use the feature request template and include all required fields.
• No system information provided — please include your OS, terminal, and OpenCode version so maintainers have context.

Please edit this issue to address the above within 2 hours, or it will be automatically closed.

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This issue might be a duplicate of existing issues. Please check:

• bug(bedrock): cachePoint on messages with document blocks causes "nothing available to cache" error #17300: Bedrock cachePoint / prompt caching on Bedrock — related Bedrock caching behavior and configuration

If you believe this was flagged incorrectly, please let a maintainer know.

[github-actions]

Thanks for updating the issue! It now follows the contributing guidelines and feature request template. Your request is clear and well-described.