fix(send): apply taproot tweak only to P2TR inputs in XprivSilentPaymentSender::send_to

[MusabYK]

Description

XprivSilentPaymentSender::send_to currently applies the BIP-341 Taproot key-path tweak to every input, regardless of the input's script type.

As a result, using XprivSilentPaymentSender::send_to with a non-Taproot input can produce an incorrect Silent Payment partial secret and therefore incorrect output keys. The call does not necessarily fail, the recipient may simply be unable to discover the payment.

The problem is that create_silentpayment_partial_secret in silentpayments/src/send/mode.rs is script-type aware. It only performs the Taproot parity handling for P2TR inputs and uses the supplied private key directly for the other eligible input types.

This PR branches on spk's actual type before deciding whether to apply the tap tweak, matching what create_silentpayment_partial_secret already expects

All Submissions:

• I've signed all my commits
• I followed the conventional commit guidelines
• I ran just p (fmt, clippy and test) before committing

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[nymius]

Can you add a test verifying this doesn't change the scanned outputs on the other end?

[MusabYK]

Hi. I've added a around-trip regression test (send_output_is_found_by_scan_with_mixed_input_types) to verify that the generated output is correctly identified by the receiver.

[nymius]

ACK 830af5a