## What changed
- Send `codex-mcp-client/<version>` as the default user agent for streamable HTTP and OAuth requests.
- Preserve user agents supplied through configured HTTP headers.
## Testing
- Verify the default user agent on OAuth discovery, token refresh, and MCP initialization requests.
- Verify that a configured user agent overrides the default.
GitOrigin-RevId: 659ef8f126df97b3c1b4d01e9e542a673b5ef42b