Fix LUT account address loading for CI

Read MXE LUT account address from artifacts/mxe_lut_acc.json at runtime
instead of hardcoding it. This ensures the correct address is used across
different environments (local vs CI) where genesis accounts are regenerated
with different addresses.

Fixes "Invalid account owner" error in GitHub Actions CI.

Author: mikemaccana

tests/arcium-solana-kit/helpers.ts

@@ -105,7 +105,21 @@ export const getMXELutAccountAddress = async (
 ): Promise<Address> => {
   // The LUT account address is loaded from artifacts/mxe_lut_acc.json as a genesis account
   // It's owned by the AddressLookupTable program, not derived as a PDA
-  return "7tYnP9JHWdZhsC1vPWpMiVCGu6StRFJJvLuN71GUFYYe" as Address;
+  // We read it from the artifact file since it changes between environments
+  const artifactPath = join(currentDir, "..", "..", "artifacts", "mxe_lut_acc.json");
+
+  try {
+    const artifactContent = await readFile(artifactPath, "utf-8");
+    const artifact = JSON.parse(artifactContent);
+    return artifact.pubkey as Address;
+  } catch (error) {
+    // If the artifact file doesn't exist yet, throw an error with a helpful message
+    throw new Error(
+      `Failed to read MXE LUT account from ${artifactPath}. ` +
+      `Ensure Arcium nodes have been initialized and genesis accounts created. ` +
+      `Original error: ${error instanceof Error ? error.message : String(error)}`
+    );
+  }
 };
 
 /**