feat: modify /plan-journey so it returns bus polylines

[jumpy-cat]

Description

Add detailed info about the path that will be taken on bus legs to make it easier for the frontend.

Type of Change

• New feature (feat)
• Bug fix (fix)
• Refactor / code improvement
• Dependency / build update
• Documentation
• Other (explain)

Related Issues

Backend Side of mbusdev/bluebus-flutter#98

Changes Made

• Flutter: n/a
• Backend (TypeScript):
  • getBusLegPolyline contains the core logic of the new feature
  • getpredictions -> processPredictions -> (various usages) refactored to add type safety. Was trying to confirm if StopTime needed rt to be optional, turns out it does since the virtual origin/dest trips don't have a route
    • refactored graphBuilder to clean up now unneeded checks and anys
    • zod schema validation done (more strict than needed, might loosen if bustimes misbehave)
  • filled out the z.any() being used for the response body schema of plan-journey
    • redid the processed leg types with zod
    • see ProcessedJourneySchema
  • consolidated LatLon type being duplicated across a few places
  • new part of transit state: stop to stop paths
    • stores a valid polyline for going from a stop to its immediate neighbors (given a route)
    • populated by buildStopToStopPaths
    • this required the Key type I made for reminders, which I refactored into the toplevel types.ts file
    • made imports from bustimeCommon follow the import * as alias pattern
• Firebase / Shared: n/a

Testing Done

Flutter:

• Tested on:
  • iOS Simulator
  • Android Emulator
  • Physical device

Screenshots / Demo (if UI or notification change)

Checklist

• Commit messages follow Conventional Commits
• PR title follows [type](scope): short description
• PR target branch is not main and is our current working update branch (e.g. maizebus2.1)
• No print() / debugPrint() / console.log() left in production code
• Secrets / keys not committed