Pre release bug fixes

[anesvijskij]

This pull request improves how backend view metadata is merged and enhances HTML validation for customization selectors. The most significant changes are the introduction of logic to merge duplicate backend views, the addition of comprehensive tests for this behavior, and expanded validation for selectors on non-field elements.

Backend Metadata Handling:

• Updated buildBackendViewMap in backend-metadata-utils.ts to merge fields from duplicate backend views while preserving the first metadata instance. This ensures that all fields from aliased or duplicate views are included without overwriting the original view metadata.
• Added mergeBackendFields helper to merge fields from duplicate views into the canonical metadata instance.
• Introduced a new test in backendMetadata.test.ts to verify that duplicate views are merged correctly and that aliases point to the canonical metadata.

HTML Validation Enhancements:

• Expanded HTML validation logic to check customization selectors on non-field elements (e.g., <qp-tab>), not just <field>, by generalizing the selector validation function. [1] [2] [3]
• Added a test case in htmlValidation.test.ts and a new fixture in SO301000_FieldSelectors.html to ensure diagnostics are produced when selectors on non-field elements reference missing targets. [1] [2]