feat(wasm-utxo): add BitGoPsbt.combineInputs for raw PSBT merge

Adds `combine_inputs` (Rust/WASM) and `combineInputs` (JS/TS) to
`BitGoPsbt`. This method merges all input fields from a raw PSBT
(provided as bytes) into the current PSBT. Unlike `combineMusig2Nonces`,
it handles all script types and works with stripped PSBTs:

- Parses the source with bitcoin's base PSBT deserializer for non-ZEC
  coins; for ZEC uses a new `ZcashBitGoPsbt::deserialize_stripped` that
  applies the ZEC-aware transaction converter but skips the
  `ZecConsensusBranchId` proprietary-key requirement.
- Dispatches on the `BitGoPsbt` enum variant so ZEC PSBTs are parsed
  correctly without requiring coin information at call sites.
- Copies per input: partial_sigs, tap_key_sig, tap_script_sigs,
  proprietary.

Used by ims-utxo to combine an HSM-signed response PSBT with the full
request PSBT before verifying and finalizing the transaction.

Tests cover BTC (p2shP2wsh), LTC, ZEC, and ZcashBitGoPsbt metadata in
a consolidated `test/fixedScript/combineInputs.ts`.

Refs: T1-3607

Author: OttoAllmendinger

packages/wasm-utxo/js/fixedScriptWallet/BitGoPsbt.ts

@@ -931,6 +931,22 @@ export class BitGoPsbt extends PsbtBase<WasmBitGoPsbt> implements IPsbtWithAddre
     this._wasm.combine_musig2_nonces(sourcePsbt.wasm);
   }
 
+  /**
+   * Merge all input fields from a raw PSBT (bytes) into this PSBT.
+   *
+   * The source bytes are parsed with the underlying bitcoin PSBT deserializer,
+   * bypassing network-specific validation (e.g. ZCash consensusBranchId), so the
+   * source may be a stripped PSBT that lacks those fields.
+   *
+   * Copies per input: partial_sigs, tap_key_sig, tap_script_sigs, proprietary.
+   *
+   * @param otherPsbtBytes - Raw bytes of the PSBT to merge signatures from
+   * @throws Error if PSBT parsing fails or input counts don't match
+   */
+  combineInputs(otherPsbtBytes: Uint8Array): void {
+    this._wasm.combine_inputs(otherPsbtBytes);
+  }
+
   /**
    * Finalize all inputs in the PSBT
    *

packages/wasm-utxo/src/fixed_script_wallet/bitgo_psbt/mod.rs

@@ -1300,6 +1300,52 @@ impl BitGoPsbt {
         Ok(())
     }
 
+    /// Merge all input fields from a raw PSBT (given as bytes) into this PSBT.
+    ///
+    /// For Zcash PSBTs the source is parsed with the ZEC-aware deserializer (skipping
+    /// the ZecConsensusBranchId check so that stripped HSM responses are accepted).
+    /// For all other coins the bitcoin PSBT deserializer is used.
+    ///
+    /// Copies per input: partial_sigs, tap_key_sig, tap_script_sigs, proprietary.
+    pub fn combine_inputs(&mut self, other_bytes: &[u8]) -> Result<(), String> {
+        let raw: Psbt = match self {
+            BitGoPsbt::Zcash(_, network) => {
+                ZcashBitGoPsbt::deserialize_stripped(other_bytes, *network)
+                    .map(|z| z.psbt)
+                    .map_err(|e| format!("Failed to parse PSBT: {}", e))?
+            }
+            _ => Psbt::deserialize(other_bytes)
+                .map_err(|e| format!("Failed to parse PSBT: {}", e))?,
+        };
+
+        let dest = self.psbt_mut();
+
+        if raw.inputs.len() != dest.inputs.len() {
+            return Err(format!(
+                "PSBT input count mismatch: source has {}, destination has {}",
+                raw.inputs.len(),
+                dest.inputs.len()
+            ));
+        }
+
+        for (src_in, dest_in) in raw.inputs.iter().zip(dest.inputs.iter_mut()) {
+            for (k, v) in &src_in.partial_sigs {
+                dest_in.partial_sigs.insert(*k, *v);
+            }
+            if let Some(sig) = src_in.tap_key_sig {
+                dest_in.tap_key_sig = Some(sig);
+            }
+            for (k, v) in &src_in.tap_script_sigs {
+                dest_in.tap_script_sigs.insert(*k, *v);
+            }
+            for (k, v) in &src_in.proprietary {
+                dest_in.proprietary.insert(k.clone(), v.clone());
+            }
+        }
+
+        Ok(())
+    }
+
     /// Serialize the PSBT to bytes, using network-specific logic
     pub fn serialize(&self) -> Result<Vec<u8>, SerializeError> {
         match self {

packages/wasm-utxo/src/fixed_script_wallet/bitgo_psbt/zcash_psbt.rs

@@ -204,10 +204,21 @@ impl ZcashBitGoPsbt {
         Ok(hash.to_byte_array())
     }
 
+    /// Deserialize a Zcash PSBT from bytes without requiring the ZecConsensusBranchId
+    /// proprietary key.  Used when combining with a stripped HSM response that may not
+    /// carry the branch ID (the key is only needed for sighash, not for merging sigs).
+    pub(crate) fn deserialize_stripped(
+        bytes: &[u8],
+        network: crate::Network,
+    ) -> Result<Self, super::DeserializeError> {
+        Self::decode_with_zcash_tx(bytes, network, false)
+    }
+
     /// Deserialize the PSBT by converting the Zcash transaction to Bitcoin format first
     fn decode_with_zcash_tx(
         bytes: &[u8],
         network: crate::Network,
+        require_branch_id: bool,
     ) -> Result<Self, super::DeserializeError> {
         let mut r = bytes;
 
@@ -342,7 +353,7 @@ impl ZcashBitGoPsbt {
         let psbt = Psbt::deserialize(&modified_psbt)?;
 
         // Consensus branch ID must be set in the PSBT proprietary map
-        if super::propkv::get_zec_consensus_branch_id(&psbt).is_none() {
+        if require_branch_id && super::propkv::get_zec_consensus_branch_id(&psbt).is_none() {
             return Err(super::DeserializeError::Network(
                 "Missing ZecConsensusBranchId in PSBT proprietary map".to_string(),
             ));
@@ -366,7 +377,7 @@ impl ZcashBitGoPsbt {
         bytes: &[u8],
         network: crate::Network,
     ) -> Result<Self, super::DeserializeError> {
-        Self::decode_with_zcash_tx(bytes, network)
+        Self::decode_with_zcash_tx(bytes, network, true)
     }
 
     /// Convert to a standard Bitcoin PSBT (losing Zcash-specific fields)

packages/wasm-utxo/src/wasm/fixed_script_wallet/mod.rs

@@ -971,7 +971,6 @@ impl BitGoPsbt {
             _ => None,
         }
     }
-
     pub fn get_outputs_with_address(&self) -> Result<JsValue, WasmUtxoError> {
         crate::wasm::psbt::get_outputs_with_address_from_psbt(self.psbt.psbt(), self.psbt.network())
     }
@@ -1816,6 +1815,20 @@ impl BitGoPsbt {
             .map_err(|e| WasmUtxoError::new(&format!("Failed to combine PSBTs: {}", e)))
     }
 
+    /// Merge all input fields from a raw PSBT (given as bytes) into this PSBT.
+    ///
+    /// The source bytes are parsed with the underlying bitcoin PSBT deserializer,
+    /// bypassing any network-specific validation, so the source may be a partial
+    /// PSBT that lacks fields like ZecConsensusBranchId.
+    ///
+    /// # Errors
+    /// Returns error if PSBT parsing fails or input counts don't match
+    pub fn combine_inputs(&mut self, other_bytes: &[u8]) -> Result<(), WasmUtxoError> {
+        self.psbt
+            .combine_inputs(other_bytes)
+            .map_err(|e| WasmUtxoError::new(&format!("Failed to combine inputs: {}", e)))
+    }
+
     /// Finalize all inputs in the PSBT
     ///
     /// This method attempts to finalize all inputs in the PSBT, computing the final