refactor: share wait_for_tx helper across Rust tutorials

[huklaa]

What changed

• Adds a shared rust_client::wait_for_tx helper for Rust tutorial binaries.
• Migrates hash_preimage_note.rs off its local duplicated transaction polling helper.
• Migrates network_notes_counter_contract.rs to the same shared helper while preserving its separate network-note polling delay.
• Migrates unauthenticated_note_transfer.rs to the shared helper as well, removing another identical polling implementation and its now-unused imports.
• Keeps runnable tutorial behavior the same while reducing repeated boilerplate.

Why

This advances #194 by moving repeated utility code into shared tutorial infrastructure so examples can focus on the protocol/client concept being demonstrated.

Scope

This PR stays focused on the duplicated wait_for_tx path and three existing runnable consumers, establishing the shared helper without mixing in broader setup refactors.

Closes part of #194.