Forward custom passthrough aggregations through stop-table APIs

[paco-barreras]

Problem

nomad.stop_detection.utils.summarize_stops supports custom aggregation of passthrough columns through passthrough_agg, but the public direct-to-stop-table APIs do not expose and forward that argument consistently. Callers therefore fall back to label APIs and external stop summarization when a stop-level field such as an attributed location_id needs modal aggregation.

Grid-Based has an additional gap: summarize_stop_grid always takes the first passthrough value, so its direct stop-table API cannot apply the same custom aggregation as other stop-detection families.

Required behavior

• Add an explicit passthrough_agg=None parameter to the public single-user direct-to-stop-table APIs and their *_per_user wrappers.
• Forward it from each per-user wrapper to its single-user stop-table function, and from there only to stop summarization.
• Do not forward summarization arguments into label APIs.
• Extend summarize_stop_grid, grid_based, and grid_based_per_user with equivalent custom passthrough aggregation behavior.
• Preserve existing first-value behavior for passthrough columns without a custom aggregator.
• Preserve canonical user_id passthrough behavior and empty stop-table schemas.

Tests

• Cover a modal nullable string and integer location_id through representative direct and per-user stop-table APIs.
• Cover Grid-Based explicitly, including populated and all-noise results.
• Verify the label APIs are not given passthrough_agg.
• Verify existing calls using only passthrough_cols remain unchanged.

This pull request includes code written with the assistance of AI. The code has not yet been reviewed by a human (remove this disclosure after human review).