dlc: build a valid script signature for P2SH-wrapped funding inputs

[bennyhodl]

Summary

Fixes the Bitcoin Red Team finding FND-004, "P2SH-P2WPKH funding inputs produce an invalid funding transaction in the stateless contract API", with a corrected root cause. The report blamed the witness-only copy in apply_funding_signatures; the final transaction is in fact rebuilt from create_dlc_transactions, which sets the script signature. The real defect sits one step earlier: redeem_script_to_script_sig wrapped the redeem script in a fresh witness program, so a 22-byte P2SH-P2WPKH redeem script failed the V0 length check and panicked inside WitnessProgram::new. Any funding input with a redeem script, including one sent by a counterparty, crashed the accepting side during fee weighting, and no P2SH input could ever fund a contract.

Changes

• redeem_script_to_script_sig now returns a single push of the redeem script, which is what spending a P2SH output requires, and returns an error instead of panicking on an oversized script. get_unsigned_tx_inputs_and_serial_ids propagates that error.
• The manager clears script signatures before building its funding PSBT, as Psbt::from_unsigned_tx demands, and restores the redeem script push on every wrapped input before extraction, so the manager path can fund from a P2SH-P2WPKH input too. Before this the manager returned "Tried to create PSBT from signed tx" for any such input.
• The stateless path needs no change beyond the helper: build_funding_psbt already strips and the finalizer already restores the push, and the final transaction is rebuilt with the correct script signature.

Testing

• New ddk-dlc unit tests: an empty redeem script yields an empty script signature; 22-byte P2SH-P2WPKH and 34-byte P2SH-P2WSH redeem scripts are pushed whole.
• New stateless test p2sh_p2wpkh_funding_inputs_carry_the_redeem_script_push: a full offer, accept, sign, and finalize cycle where the offerer funds from a wrapped output. The funding transaction's wrapped input carries the redeem script push and a two-element witness, native inputs stay empty. The harness's funding transaction check now understands wrapped inputs.
• cargo test -p ddk-dlc (22), the stateless suite (33), and the regtest enum_single_oracle_test pass. cargo check --all-features --tests and cargo +1.89.0 clippy -- -D warnings are clean.