[FEATURE] Add Bedrock structured output support

[llenodo]

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?

#608 adds structured output support for the direct Anthropic API. Bedrock's Converse API also supports structured output but uses a different payload format — the schema must be stringified JSON nested under outputConfig.textFormat.structure.jsonSchema. Currently with_schema is not wired up for the Bedrock provider.

Proposed solution

Wire outputConfig into Bedrock's render_payload when a schema is provided, add supports_structured_output? to Bedrock model capability detection (Claude 4.5+), and add unit and integration tests with a VCR cassette.

Why this belongs in RubyLLM

Consistent support for structured outputs across providers — Bedrock is a core provider that should have parity with the direct Anthropic API.

PR: #619

[llenodo]

I have a PR ready but closed it per the Contributing guidelines. LMK and I can open it up

[llenodo]

Closing and rercreating so we get the proper 'enhancement' label