feat: add AWS Bedrock as a selectable chat LLM transport

[hweej]

Summary

• Enable AWS Bedrock as a selectable chat transport (CHAT_LLM_TRANSPORT=bedrock) — the bedrock branch of AnthropicLLMClient already existed but was unusable.
• Add the anthropic[bedrock] (boto3) runtime dependency, required for Bedrock SigV4/credential resolution.
• Add explicit CHAT_BEDROCK_REGION config (default us-east-1), threaded into AsyncAnthropicBedrock(aws_region=...).
• Direct-Anthropic path is the default and unchanged — no behavior change until a deployment opts in.

Test Plan

• agent suite: 268 passed; api chat_session tests: 15 passed
• Running prod on Bedrock is deferred — needs the AWS-credentials mechanism + deploy config (CHAT_LLM_TRANSPORT, CHAT_LLM_MODEL, CHAT_BEDROCK_REGION)