Fix ValueError from invalid CSS selectors in EmbedAPI (#12573)
This PR adds defensive handling for invalid CSS selectors in the
EmbedAPI to prevent crashes.
## Changes
- Added try-except blocks around calls to catch exceptions
- Invalid selectors now log a warning and return None instead of
crashing
- Applies to both the query parameter and fragment-based selectors
## Issue
Fixes: https://read-the-docs.sentry.io/issues/6923723755/
Previously, when malformed CSS selectors like `@@R06GA` were passed,
selectolax would raise a ValueError that wasn't caught, causing the
entire request to fail.