Clarify PSBT witness test comment to reflect actual Taproot support boundaries

[Copilot]

The PSBT test comment implied P2TR was generally unsupported, which is inaccurate given existing key-path support in PSBT handling and tx building. This updates the test annotation to document the real current limitation: unsupported witness paths are P2WSH and P2TR script-path.

• Comment accuracy update (PSBT witness path)

  • Reworded the inline comment in rust/tw_tests/tests/chains/bitcoin/bitcoin_sign/psbt.rs to avoid conflating all P2TR with unsupported behavior.
  • Kept scope specific to the tested path (P2WPKH witness UTXO) and explicitly called out unsupported cases.

• Behavioral scope documented

  • Clarifies that the limitation is not Taproot key-path itself, but Taproot script-path (and P2WSH) in this PSBT flow.

// This test path uses a supported witness UTXO script type (P2WPKH).
// Please note that `P2WSH` and `P2TR` script-path witness inputs are not supported yet.