feat(providers): add AWS Bedrock provider (#1903)

Add support for AWS Bedrock as an LLM provider using the Converse API.
The implementation is behind a build tag (-tags bedrock) to keep the
default binary size small.

Features:
- AWS SDK v2 with automatic credential chain (env vars, profiles, IAM roles)
- Converse API for unified access to Claude, Llama, Mistral models
- Tool/function calling support with proper document handling
- Image support with base64 decoding and size limits
- Request timeout configuration
- Region validation and endpoint resolution for all AWS partitions

Usage:
  go build -tags bedrock
  model: bedrock/us.anthropic.claude-sonnet-4-20250514-v1:0
  api_base: us-east-1  (or full endpoint URL)

go.sum

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