feat: Bulk reject remaining applications on admin rejection

[lauracabtay]

Description

A mentee can submit applications to multiple mentors in a single cycle, prioritised by order. When an admin rejects a mentee's application (e.g. the mentee is not eligible for the programme), their other open applications in that cycle - those still in PENDING or MENTOR_REVIEWING status - remain open unnecessarily. These should be rejected in bulk as part of the same action.

Requirements

• When an admin rejects a mentee application, all other applications for that mentee in the same cycle that are in PENDING or MENTOR_REVIEWING status must also be automatically rejected.
• The bulk rejection must use the same reason provided in the original rejection request.
• The entire operation must be atomic (single transaction).
• Applications already in terminal states (REJECTED, DROPPED, MENTOR_ACCEPTED, MENTOR_DECLINED) must not be affected.

Acceptance Criteria

• Given a mentee with multiple open applications in a cycle, when admin rejects one, then all other PENDING and MENTOR_REVIEWING applications for that mentee in that cycle are also rejected with the same reason.
• Given a mentee with only one application, when admin rejects it, then only that application is rejected (no error occurs).
• Given a mentee with applications where some are already in terminal states, when admin rejects one, then only the non-terminal ones are additionally rejected.