fix(bedrock): forward default model from provider

[ShiroKSH]

Summary

• Forward the model parsed by the Bedrock auto-client to the Bedrock client factory.
• Add a regression assertion for the forwarded default model.

Root cause

The Bedrock auto-client selected a mode from the model string but never passed that string as the factory's default model. Calls that omitted model therefore reached Bedrock without the required modelId.

Validation

• Targeted auto-client and Bedrock coverage tests: 31 passed.
• Ruff check for the two changed files passed.

[jxnl]

Consolidated and shipped in #2495. Closing this focused patch as superseded; thank you for the contribution.