Support "extended wall-clock time" for schedules (e.g., GTFS 25:10) without breaking RFC3339 validators

### Problem

Beckn fields currently use **RFC3339 `date-time`**. For **date-agnostic schedules** (recurring services), mobility needs **service-day wall-clock times** that can exceed 24 hours (e.g., **`25:10` = 01:10 next day**, GTFS-style). RFC3339 cannot represent this.

### Ask

Propose a **standard representation** for schedule times that:

* supports **hours > 23** for service-day rollover (e.g., `25:10`, `49:05`)
* is **domain-agnostic** and interoperable
* provides a **smooth migration path** for implementers who already strictly validate RFC3339 (e.g., backward-compatible transition / dual-acceptance / capability signaling)

### Must-support examples

* Daily departure: `15:00`
* Arrival next day: `25:10`
* Multi-day rollover: `49:05`

### Outcome

Agree on a representation + minimal migration guidance, then document it in the bindings with test vectors.