Probe .idx instead of .sub for external VobSub subtitle language detection

External VobSub subtitle pairs (.idx and .sub) were only probed via the
bare .sub file. In cases where multiple languages are present, this
results in missing language metadata.

Fix by detecting the matching .idx file during media info resolution
to run ffprobe on that file and skip processing the .sub entirely.
ffprobe will automatically find the matching (same directory,
case-sensitive base) .sub file and process both.

Added regression tests covering idx/sub pairing, unpaired files,
cross-directory pairs, and language-flagged filenames.

Fixes #17745

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