`sign_input` should respect `allow_all_sighashes` sign option

[ValuedMammal]

Description

SignOptions::allow_all_sighashes is documented as controlling whether the signer will accept non-SIGHASH_ALL sighash types. However, this flag is only checked inside Wallet::sign as a PSBT-wide pre-flight guard. The underlying InputSigner::sign_input never consults it although it's a public trait method. Callers that use InputSigner or TransactionSigner directly bypass the check entirely.

Affected code

• SignerWrapper<PrivateKey>::sign_input — the leaf implementation for all BDK software signers.
  DescriptorXKey and DescriptorMultiXKey both derive a key and then delegate here, so all three are affected through the same code path.
• All signing contexts (Legacy, Segwitv0, Taproot) are affected.

The ECDSA path calls psbt.sighash_ecdsa() which returns whatever type the PSBT declares and proceeds. The Taproot path calls compute_tap_sighash() which similarly reads the PSBT sighash type without checking allow_all_sighashes.

Steps to reproduce

1. Obtain a signer directly (e.g. from wallet internals or a custom SignersContainer)
2. Set SIGHASH_NONE on a PSBT input
3. Call signer.sign_input(&mut psbt, 0, &SignOptions::default(), &secp) (signs successfully despite allow_all_sighashes being false)

Expected behavior

sign_input should return Err(SignerError::NonStandardSighash) when sign_options.allow_all_sighashes is false and the PSBT input declares a sighash type other than SIGHASH_ALL / SIGHASH_DEFAULT.

Proposed fix

Add the sighash guard in SignerWrapper<PrivateKey>::sign_input, after the "already finalized" early return but before the match self.ctx dispatch. This ensures the check covers all signing contexts and all delegating implementations in a single place:

if !sign_options.allow_all_sighashes {
    let sighash_type = psbt.inputs[input_index].sighash_type;
    if sighash_type.is_some()
        && sighash_type != Some(EcdsaSighashType::All.into())
        && sighash_type != Some(TapSighashType::All.into())
        && sighash_type != Some(TapSighashType::Default.into())
    {
        return Err(SignerError::NonStandardSighash);
    }
}

This mirrors the existing logic in Wallet::sign. Through the Wallet::sign path, behavior is unchanged since the PSBT-wide check still fires first.

A test should be added covering:

• Taproot input with SIGHASH_NONE and allow_all_sighashes: false returns NonStandardSighash
• The same with allow_all_sighashes: true proceeds normally.