Fix metadata updater JSON handling

[fallintoplace]

Summary

• Use Python JSON serialization when formatting metadata keys, string values, and array items.
• Exit with an error when a requested metadata file has no matching target to update.

Root cause

The metadata updater manually wrapped strings in quotes while preserving the generated Jackson-like layout. That could emit invalid JSON when existing metadata values or new service versions contained characters that need escaping. The script also ignored update results, so a missing match could leave generated metadata stale while the workflow continued.

Validation

• python3 -m py_compile .github/actions/check-container-upgrade/update-metadata-version.py
• git diff --check
• Ran temp-file checks covering quoted/backslash/newline values, successful updates across both metadata files, and the no-match failure path.

[github-actions]

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[davsclaus]

Clean fix — replacing manual string formatting with json.dumps() properly handles escaping for special characters, and the no-match error exit is the right behavior for a CI script. LGTM.

Minor note: commit title doesn't follow the CAMEL-XXXX: convention, but this is a CI infrastructure fix so it's not a blocker.

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