Improve error handling and fix format issues

[murshed]

Fix DRF serializer error format, oEmbed exception handling, shell JSON escaping, and skip button focus CSS

Description

This pull request addresses several reliability, compatibility, and accessibility issues across the repository:

• Fix serializer error format to be DRF-compliant in OembedRequestSerializer:
  Updated the ValidationError error dictionary structure from {"Could not parse identifier from URL.": data["url"]} to standard DRF-compliant structure {"url": ["Could not parse identifier from URL."]} in api/api/serializers/image_serializers.py. This aligns with the OpenAPI specification (image_oembed_400_example) and standard PHP / WordPress client expectations.

• Add error handling and logging for image dimension fetching in oEmbed endpoint:
  Wrapped out-of-band image downloading and PIL image dimension parsing in a try...except block in api/api/views/image_views.py. Structured warnings (failed_to_get_image_dimensions_for_oembed) are logged when network or parsing errors occur, preventing unhandled HTTP 500 responses.

• Add HTTP status check before processing image content:
  Added explicit image_file.status == 200 verification before reading image content bytes and attempting PIL dimension extraction in image_views.py.

• Escape special characters in dag-sync.sh JSON payload for robustness:
  Added parameter expansion rules ${subject//\\/\\\\} and ${subject//\"/\\\"} in dag-sync.sh to escape backslashes and double quotes in commit messages before constructing Slack webhook JSON payloads.

• Update skip button CSS selector for better browser compatibility:
  Updated the CSS selector in frontend/src/components/VSkipToContentButton.vue to .skip-button:not(:focus, :focus-visible) so the skip-to-content button is visually displayed whenever focused across all browser focus modes.

Testing Instructions

1. API / oEmbed Serializer & View:
   • Query v1/images/oembed/ with an invalid url parameter (e.g., ?url=invalid) and verify that the 400 response body is formatted as {"url": ["Could not parse identifier from URL."]}.
   • Query v1/images/oembed/ for an image whose remote URL returns 404 or times out, and verify that the view returns a valid 200 oEmbed response without throwing a 500 Internal Server Error.
2. Shell Script:
   • Run bash -n dag-sync.sh to verify syntax correctness.
3. Frontend:
   • Tab into the frontend application and verify that the "Skip to content" button displays properly upon receiving focus.

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
• My pull request targets the default branch of the repository (main) or a parent feature branch.
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added or updated tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no visible errors.
• I ran the DAG documentation generator (ov just catalog/generate-docs for catalog PRs) or the media properties generator (ov just catalog/generate-docs media-props for the catalog or ov just api/generate-docs for the API) where applicable.

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