fix: prevent duplicate keys in MetadatumMap arbitrary generator

The property-based test generator for MetadatumMap was creating
invalid metadata by allowing duplicate keys (e.g., two empty strings).
When duplicate keys were added to the Map, the second insertion
would overwrite the first, causing a mismatch between the expected
array length and actual Map size.

This caused flaky test failures in AuxiliaryData.CML.test.ts when
comparing Evolution SDK CBOR output with CML CBOR output.

Fixed by using FastCheck.uniqueArray with a selector function that
ensures unique keys based on their string values.

Fixes intermittent CI failures in test/AuxiliaryData.CML.test.ts

Author: solidsnakedev

packages/evolution/src/core/TransactionMetadatum.ts

@@ -258,12 +258,15 @@ export const arbitrary: FastCheck.Arbitrary<TransactionMetadatum> = FastCheck.on
     ),
     { maxLength: 3 }
   ).map((value) => new ArrayMetadatum({ value })),
-  FastCheck.array(
+  FastCheck.uniqueArray(
     FastCheck.tuple(
       FastCheck.string().map((value) => new TextMetadatum({ value })),
       int64Arbitrary.map((value) => new IntMetadatum({ value }))
     ),
-    { maxLength: 3 }
+    {
+      maxLength: 3,
+      selector: ([key]) => key.value // Ensure unique keys by their string value
+    }
   ).map((entries) => {
     const map = new Map()
     for (const [key, value] of entries) {