fix(noq): resolve pending OpenPath on connection termination

[mp0rta]

This was discovered while testing an experimental Zenoh-over-MPQUIC demo:
when the underlying QUIC connection was closed while an additional path was still being validated, the task awaiting open_path() never completed, stalling transport shutdown and recovery.

Description

A pending OpenPath future did not resolve when its connection terminated.

State::terminate notified the other waiter classes but left the
open_path watch senders registered. As a result, the future could remain
pending indefinitely and retain the connection state through its
ConnectionRef.

This change:

• drains pending open_path senders during connection termination;
• resolves them with PathError::ValidationFailed, matching the existing
  handling of PathEvent::Abandoned for paths that have not opened yet;
• adds a regression test that closes a connection while path validation
  against a blackholed address is pending.

Related to #684.

Breaking Changes

None.

Notes & open questions

The regression test passes on the updated implementation and would time out
without the termination handling.

Change checklist

• Self-review.
• Tests if relevant.
• All breaking changes documented.
• This PR was created by a human that thought critically about the
  proposed change and wrote as clear and concise a description as possible.
• This PR isn't slop and is carefully crafted to have the intended effect.
• cargo make passes locally.