Motivation

• Replace assumption-based input rejection in a proptest with a generator that yields only valid (distinct) label pairs to make the property clearer and avoid runtime prop_assume! filtering.

Description

• Add a distinct_labels() helper that returns a (String, String) pair strategy and uses .prop_filter() to ensure the two labels differ.
• Gate the helper with #[cfg(feature = "jwk")] to avoid dead-code warnings when the jwk feature is not enabled.
• Update the different_labels_produce_different_kids property to consume (label1, label2) in distinct_labels() and remove the inline prop_assume!.

Testing

• Ran cargo test -p uselesskey-ecdsa --test ecdsa_prop and the proptest suite completed successfully (4 passed; 0 failed).

Codex Task