feat: add AWS Bedrock provider support

[leocavalcante]

Summary

Adds multi-provider support allowing the proxy to route requests to either GitHub Copilot API or AWS Bedrock based on the model name.

Routing Logic

• Opus models → GitHub Copilot API
• Sonnet/Haiku models → AWS Bedrock (if configured)

If AWS Bedrock credentials are not configured, all models automatically fall back to GitHub Copilot API.

Configuration

AWS Bedrock is configured via environment variables:

export AWS_REGION=us-east-1
export AWS_ACCESS_KEY_ID=your_key_id
export AWS_SECRET_ACCESS_KEY=your_secret_key

Implementation Details

• ✅ AWS Bedrock Converse API integration
• ✅ Model-based provider routing (transparent to clients)
• ✅ Support for both streaming and non-streaming responses
• ✅ Automatic translation between OpenAI/Anthropic and Bedrock formats
• ✅ Graceful fallback to Copilot if Bedrock not configured

Files Changed

• src/services/bedrock/create-chat-completions.ts - New Bedrock service
• src/services/provider-router.ts - Provider routing logic
• src/lib/state.ts - Added AWS credentials to state
• src/start.ts - Load AWS credentials from environment
• src/routes/*/handler.ts - Updated to use provider router

Test Plan

• Test with Opus model → should route to GitHub Copilot
• Test with Sonnet model → should route to AWS Bedrock (if configured)
• Test with Haiku model → should route to AWS Bedrock (if configured)
• Test fallback when AWS not configured
• Test both streaming and non-streaming modes