fix(roadtrip): one ride-gap rule for both corridor searches, and two seam edges

The corridor search in the browser cut its tiles around a ride, the MCP tool
did not: it tiled and measured along the straight line between two terminals,
so an assistant searching a flight day got boxes and hits under the flight
path. The gap logic moves into shared/roadtrip/corridor.ts and both sides use
it. A departure less than the check-in allowance after midnight no longer
pins on the previous evening, and a hire car whose pick-up desk has no
position is not seamed at its return desk.

shared/src/maps/maps.schema.ts

@@ -30,7 +30,9 @@ export const mapsSearchRequestSchema = z.object({
    * Ask one provider alone for this search. The index and OpenStreetMap answer
    * first by default and Google is only asked when they find nothing; a caller
    * whose results were not the place they meant can send the same query to
-   * Google instead. Ignored on an instance where Google holds no key.
+   * Google instead. Ignored unless Google holds the keyed slot: without a
+   * Google key, or with Amap or OpenStreetMap picked as the places provider,
+   * the index and OpenStreetMap answer as usual.
    */
   provider: z.enum(['google']).optional(),
 });