fix: prevent duplicate keys in MetadatumMap arbitrary generator

[solidsnakedev]

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

[Copilot]

Pull Request Overview

Fixes a test flakiness issue caused by duplicate keys in the MetadatumMap property-based test generator. The generator was creating invalid metadata maps where duplicate keys would overwrite each other, causing mismatches between expected and actual map sizes during CBOR serialization comparisons.

• Changed from FastCheck.array to FastCheck.uniqueArray to prevent duplicate keys
• Added a selector function that ensures uniqueness based on the TextMetadatum string values

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