fix: persist block media selections

[bobbwal]

Description

Ensure media selections inside block/object fields persist by dispatching input/change events when media values update, and add E2E coverage.

Fixes #644

Changes

• Dispatch input/change events for media field updates (select/remove/clear) to keep structured fields in sync.
• Add E2E coverage for media persistence in content blocks.

Testing

• npx playwright test tests/e2e/53-block-media-persist.spec.ts --config tests/playwright.config.ts

Unit Tests

• Added/updated unit tests
• All unit tests passing

E2E Tests

• Added/updated E2E tests
• All E2E tests passing

Checklist

• Code follows project conventions
• Tests added/updated and passing
• Type checking passes
• No console errors or warnings
• Documentation updated (if needed)

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