Add unit tests for reportUnmatchedStopClusters

[Vishmayraj]

stop_clusters.go was the only file in internal/metrics/ without a
corresponding test file.

This PR adds stop_clusters_test.go covering the following cases:

• Empty stop map reports no metrics
• Stop with no parent and valid coordinates uses S2 clustering
• Station type stop clusters by its own ID
• Malformed stop with no parent and no coordinates is skipped
• Multiple stops in the same station cluster are aggregated correctly

All existing tests continue to pass.

[CLAassistant]

All committers have signed the CLA.