Make subsonic provider check for extension it uses

[khers]

We recently bumped into a problem with a new Open Subsonic implementation that does not support the 'formpost' extension. This relies on that extension and should check for it early and fail early if it is not available.

Related issue (if applicable):

• related issue OpenSubsonic Media Server Library application/xml not supported. support#5877

Types of changes

• Bugfix (non-breaking change which fixes an issue) — bugfix
• New feature (non-breaking change which adds functionality) — new-feature
• Enhancement to an existing feature — enhancement
• New music/player/metadata/plugin provider — new-provider
• Breaking change (fix or feature that would cause existing functionality to not work as expected) — breaking-change
• Refactor (no behaviour change) — refactor
• Documentation only — documentation
• Maintenance / chore — maintenance
• CI / workflow change — ci
• Dependencies bump — dependencies

Checklist

• The code change is tested and works locally.
• pre-commit run --all-files passes.
• pytest passes, and tests have been added/updated under tests/ where applicable.
• I have read and complied with the project's AI Policy for any AI-assisted contributions.

[khers]

Do I need to do anything more to enable the translation of the two new exception messages?

[OzGav]

To make them translatable you would add this or similar

raise LoginFailed(
    msg,
    translation_key="extensions_query_failed",
    translation_owner=self.translation_owner,
) from e

and then in strings.json errors block add

  "extensions_query_failed": "Failed to query server for OpenSubsonic extensions",
  "formpost_not_supported": "Server does not support the 'formpost' OpenSubsonic extension, which is required."

[khers]

Thanks, I will add that to the PR when I get home this evening.

[khers]

Translation entries should be all set now.

[OzGav]

Perfect @khers !