Add OpenAPI tag-filter endpoint and fix local resource path

[Theosakamg]

Summary

Add a new endpoint to return a filtered OpenAPI document by tag and fix local raw resource path resolution that caused false 404 errors for OpenAPI files.

Changes

• Add GET /openapi-spec/tag/:tag
• Filter OpenAPI paths/operations by selected tag (case-insensitive)
• Return 404 when tag is not found or has no linked endpoints
• Preserve JSON/YAML behavior and cache headers
• Refactor OpenAPI route helpers for format/file resolution
• Fix ResourceManager local raw path resolution for:
  • getLocalResourceRaw
  • getLocalResourceRawWithMetadata

Validation

• npm run build (backend) passes
• Local raw OpenAPI file read smoke test passes

Notes

• Commit includes only backend route and resource manager changes.

[flovntp]

Suggested change

	if (sdks) {
	return 'openapispec-upsun-sdks.json';
	}
	return format === 'yaml' ? 'openapispec-upsun.yaml' : 'openapispec-upsun.json';
	const filename = sdks ? 'openapispec-upsun-sdks' : 'openapispec-upsun';
	return format === 'yaml' ? `${filename}.yaml` : `${filename}.json`;