aws bedrock bearer

[alex-dev1]

• add bedrock as a supported provider in the CLI setup and selector flows
• support AWS Bedrock bearer-token authentication for bedrock:* models
• infer AWS region from standard Bedrock runtime endpoints when base_url is provided
• apply the Bedrock auth setup consistently across chat, structured output, RLM calls, and embeddings
• document the Bedrock setup flow in the README, including the required embedding configuration
• add unit tests for Bedrock env and region handling