[FEATURE] Add Structured Output for Amazon Bedrock

[aniamisiorek]

Scope check

• This is core LLM communication (not application logic)
• This benefits most users (not just my use case)
• This can't be solved in application code with current RubyLLM
• I read the Contributing Guide

Due diligence

• I searched existing issues
• I checked the documentation

What problem does this solve?

Currently, we're doing the work-around solution for structured output for Amazon Bedrock models, but structured output would mitigate this need.

Proposed solution

Add structured output to Amazon Bedrock via render_payload. Structured output is supported as of 02-04-2026 (https://docs.aws.amazon.com/bedrock/latest/userguide/structured-output.html).

Why this belongs in RubyLLM

Structured output is currently supported for a few providers--it would be great to get Amazon Bedrock as well.