Use mergeInvitationCommits in resolveCommitReferences for correct commit merging.

Delegate input/output merging to the engine so mergesWith extensions and
transaction indices resolve correctly instead of flattening raw commits.

tests/utils/resolve-invitation-data.test.ts

@@ -123,6 +123,36 @@ const originalInvitation: XOInvitation = {
   ],
 };
 
+/**
+ * Customer input commit extended with unlocking bytecode via mergesWith (signing flow).
+ */
+const invitationWithSignedInput: XOInvitation = {
+  ...originalInvitation,
+  commits: [
+    ...originalInvitation.commits.slice(0, 5),
+    {
+      commitIdentifier: "a1b2c3d4e5f6a7b8c9d0e1f2a3b4c5d6",
+      previousCommitIdentifier: "d18b9a0caa638eaa1d0711f333e9c114",
+      entityIdentifier: CUSTOMER_ENTITY,
+      data: {
+        inputs: [
+          {
+            mergesWith: {
+              commitIdentifier: "d18b9a0caa638eaa1d0711f333e9c114",
+              index: 0,
+            },
+            unlockingBytecode:
+              "41226b2be7c2890c8bbde2f79e79640e56d866843f2e822ec51c469019d13db0",
+          },
+        ],
+      },
+      signature:
+        "3045022001a1b2c3d4e5f6789012345678901234567890abcdef1234567890abcdef123456789022100fedcba0987654321fedcba0987654321fedcba0987654321fedcba0987654321",
+      expiresAtTimestamp: 1779507008000,
+    },
+  ],
+};
+
 describe("resolveCommitReferences", () => {
   it("flattens commits and enriches items with template metadata", () => {
     const resolved = resolveCommitReferences(
@@ -237,4 +267,21 @@ describe("resolveCommitReferences", () => {
     expect(resolved.outputs[1]).not.toHaveProperty("name");
     expect(resolved.outputs[1]).not.toHaveProperty("outputIdentifier");
   });
+
+  it("merges input extension commits via mergesWith into a single input", () => {
+    const resolved = resolveCommitReferences(
+      invitationWithSignedInput,
+      vendingMachineTemplate,
+    );
+
+    expect(resolved.inputs).toHaveLength(1);
+    expect(resolved.inputs[0]).toMatchObject({
+      entityIdentifier: CUSTOMER_ENTITY,
+      outpointTransactionHash:
+        "b1e8f77cdc60efac19f668fc5c7177ace42a46e2532f230979559c7190c3c80a",
+      outpointIndex: 1,
+      unlockingBytecode:
+        "41226b2be7c2890c8bbde2f79e79640e56d866843f2e822ec51c469019d13db0",
+    });
+  });
 });