fix(bedrock): support region and bearer token configuration

[wnkz]

Fixes #5812

Add support for configuring Amazon Bedrock provider region and bearer token through opencode.json and auth.json.

Region Configuration

The amazon-bedrock provider now respects the region option from config (opencode.json) with proper precedence:

1. options.region from opencode.json (highest priority)
2. AWS_REGION environment variable
3. Default us-east-1 fallback

Users can now configure:

{
  "provider": {
    "amazon-bedrock": {
      "options": { "region": "eu-west-1" }
    }
  }
}

Bearer Token from Auth

The provider now checks ~/.local/share/opencode/auth.json for bearer tokens with precedence:

1. AWS_BEARER_TOKEN_BEDROCK environment variable (if set)
2. Bearer token from auth.json (if available)

This allows users to configure their Bedrock bearer token once and have it automatically used by OpenCode.

Changes

• Remove Promise.all() from env var retrieval and add Auth.get() check for bearer tokens
• Implement bearer token precedence: env var first, then auth.json
• Rename region to defaultRegion to clarify it's the default used when no config override exists
• Add region variable in getModel() for resolved region with config precedence
• Update getModel() parameter from _options to options (was previously ignored)
• Add region resolution precedence comment and tests for region and auth configuration scenarios

[rekram1-node]

/review