Refacto work_schedule explainer

[ppitou]

Description and goal

Follow up of #17938

• Remove the optional on partial_pathfinding_result and last_reached_operational_point core, editoast: enable work schedules failure explainer  #17938 (comment)
• Factorize fetch_work_schedule and fetch_operational_point into a function to avoid having duplicated code, as they will be used in the same way/order for both most_blocking and nearest work schedules core, editoast: enable work schedules failure explainer  #17938 (comment)
• Returns all the results that core has, and make the front decide how much it shows. At the very least, that would explain better why nearest_to_destination_work_schedules is a Vec even if it always contain one element. But in general, I believe it’s really a UX decision to show only a few, so these thresholds should be encoded in the front core, editoast: enable work schedules failure explainer  #17938 (comment)

Acceptance criteria

A list of conditions which define when this task shall be considered complete