address review feedback: surface bedrock credential errors, validate static creds, docs, tests
- ResolveBedrockCredentials errors are now returned by New() instead of being ignored
- Static credentials now require both aws_key and aws_secret; partial configs return an error
- Update config.example.yaml and docs/configuration.md with aws_profile and aws_session_token
- Add bedrock_credentials_test.go covering nil config, partial static credentials,
valid static credentials, default credential chain, and nonexistent profiles