feat: add ProbeHandle and probe success/failure events

[pwltr]

Description

This PR exposes the probe lifecycle end-to-end, allowing consumer apps (Bitkit) to actually use the result of a probe.

The node’s event queue now surfaces Event::ProbeSuccessful and Event::ProbeFailed (mapped from LDK’s probe events), and send_probes / send_probes_using_amount (BOLT11) plus send_probes (spontaneous) return a ProbeHandle per path with the same payment_id and synthetic payment_hash, so apps can match handles to those events reliably.

UniFFI exposes ProbeHandle as a dictionary in ldk_node.udl, following the same pattern as other records (hand-written Rust struct + pub use before include_scaffolding!). Swift bindings are updated accordingly.

How to use (consumer): keep a map of pending probes keyed by payment_id from the returned handles; on ProbeSuccessful / ProbeFailed, match payment_id (and optionally payment_hash) and remove the entry.

[pwltr]

Currently creating bindings... Otherwise ready for review.

Should we aim to upstream this or not necessary?