Add tests for `MissingNonWitnessUtxo` / `InvalidNonWitnessUtxo` signing errors

**Describe the enhancement**

`Wallet::sign_with_signers` returns `SignerError::MissingNonWitnessUtxo` when a non-taproot input lacks `non_witness_utxo` and `SignOptions::trust_witness_utxo` is `false` (`src/wallet/mod.rs`, signing loop), and `SignerError::InvalidNonWitnessUtxo` when the provided one does not match the outpoint. Neither branch is covered by tests: nothing in `tests/` or the unit tests signs a segwit v0 PSBT without `non_witness_utxo` under default options, or with a mismatching `non_witness_utxo`.

Since this check is what protects users from signing with a wrong input amount, regressions there would currently go unnoticed.

This issue was found by AI.

Suggested coverage:
- Segwit v0 PSBT with `non_witness_utxo = None`, default `SignOptions` → `MissingNonWitnessUtxo`.
- Same PSBT with `trust_witness_utxo: true` → signs.
- `non_witness_utxo` whose txid does not match the outpoint → `InvalidNonWitnessUtxo`.

**Use case**

Regression protection for the signing path.

**Impact**
- [x] Developer experience / maintainability

**Which backend(s) are relevant (if any)?**
- [x] None / not backend-related (e.g. `bdk_chain`, `bdk_core`)

Start with the signing loop in src/wallet/mod.rs, then inspect existing signing tests in tests/ and the unit tests for PSBT setup patterns. Add coverage for missing and mismatching non_witness_utxo values, including the trust_witness_utxo case, and run the relevant wallet test suite to confirm the expected errors and successful signing.

Written by the indexing model from the issue text.