Support AWS Bedrock out of the box

[julian-michelmann]

Description
The agent currently connects to Bedrock only via LiteLLM. For many users it would be easier to use Bedrock directly without extra layers.

Change

• Add native Bedrock integration as a first-class option.
• Keep LiteLLM support as an alternative.

Why

• Simplifies setup for teams already on AWS.
• Reduces dependencies and potential points of failure.

Tasks

• Implement Bedrock SDK client support.
• Add config option to use bedrock.
• Update readme