Previously, external schema references (URLs) in OpenAPI schemas were
silently passed through and only failed during JSON schema validation
with confusing "failed to match exactly one schema" errors.
This change:
- Detects external references in _replace_ref_with_defs() and raises clear error messages
- Updates exception handlers to propagate external reference errors while preserving other error handling
- Adds comprehensive test coverage for external reference detection
- Provides helpful error messages explaining that FastMCP only supports local schema references
Fixes#926🤖 Generated with [Claude Code](https://claude.ai/code)
Co-Authored-By: Claude <noreply@anthropic.com>
