refactor: extract ParseLocationParams helper to remove duplication across location handlers

[AhmedAlian7]

Description

Extracts the duplicated location query parameter parsing logic shared across
three handlers into a single reusable helper.

Closes #745

Changes

internal/utils/api.go

• Added LocationParams struct holding Lat, Lon, LatSpan, LonSpan, Radius
• Added ParseLocationParams(q url.Values) which reuses existing ParseRequiredFloatParam
  for lat/lon (required) and ParseFloatParam for the optional fields
• Returns nil struct pointer on any validation error, consistent with existing error
  accumulation pattern

internal/restapi/stops_for_location_handler.go

• Replaced 5 individual Parse*FloatParam calls with a single utils.ParseLocationParams call

internal/restapi/routes_for_location_handler.go

• Same replacement as above

internal/restapi/trips_for_location_handler.go

• Replaced 4 Parse*FloatParam calls in parseAndValidateRequest with
  utils.ParseLocationParams
• locParams.Radius is intentionally ignored — this handler does not use it

internal/utils/api_test.go

Added 8 test cases for ParseLocationParams:

Testing

make test
make lint
go fmt ./...

[aaronbrethorst]

@AhmedAlian7 please fix the merge conflicts!