fix(wallet): make sign()'s sighash guard context-aware

[muhahahmad68]

Fixes #519.

Description

The pre-flight sighash guard in Wallet::sign() checked every PSBT input against a flat set of allowed sighash values without regard to whether that input was actually Taproot. As a result, TapSighashType::Default (0x00) was accepted unconditionally for every input, including Legacy/Segwitv0 ones, where it has no valid ECDSA meaning.

This is currently masked for the built-in software signer, since sign_input()'s guard (introduced in #476) independently rejects it downstream. However, Wallet::sign() dispatches to any TransactionSigner, and the whole point of this pre-flight guard is that individual signers can trust it has already filtered non-standard sighashes.

Notes to the reviewers

Infer Taproot-ness per input from witness_utxo.script_pubkey, consistent with how complete_transaction() determines Taproot-ness for foreign UTXOs, and apply the correct sighash check for that input's context.

Tests added:

• sign_rejects_ecdsa_input_with_taproot_default_sighash — confirms a non-Taproot input carrying TapSighashType::Default is now rejected
• sign_allows_taproot_input_with_default_sighash — confirms a genuine Taproot spend still signs and finalizes normally

Before submitting

• I followed the contribution guidelines
• This PR breaks the existing API

[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 81.86%. Comparing base (e13e8a0) to head (3956e04).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/wallet/mod.rs	91.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #528      +/-   ##
==========================================
+ Coverage   81.84%   81.86%   +0.01%     
==========================================
  Files          25       25              
  Lines        6487     6494       +7     
  Branches      296      294       -2     
==========================================
+ Hits         5309     5316       +7     
- Misses       1078     1079       +1     
+ Partials      100       99       -1     

Flag	Coverage Δ
rust	81.86% <91.66%> (+0.01%)	⬆️

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[ValuedMammal]

One suggestion I have for determining whether an input is a p2tr spend is it may be safer and more reliabe to test witness_utxo.script_pubkey.is_p2tr() rather than the tap_* metadata fields (at least that's consistent with how the check is handled elsewhere e.g. complete_transaction).

[muhahahmad68]

One suggestion I have for determining whether an input is a p2tr spend is it may be safer and more reliabe to test witness_utxo.script_pubkey.is_p2tr() rather than the tap_* metadata fields (at least that's consistent with how the check is handled elsewhere e.g. complete_transaction).

Good catch