Implement proper Arbitrary trait for ZkLoginInputs property testing

[Copilot]

The Arbitrary implementation for ZkLoginInputs used hardcoded test values instead of generating them dynamically, limiting property-based test coverage.

Changes

• ZkLoginClaim generation: Generates random issuer values, creates properly formatted extended claims ("iss":"https://<random>.example.com",), and encodes as base64url. Uses index_mod_4 = 0 to avoid bit padding complexity.

• JWT header generation: Dynamically creates and base64url-encodes valid JWT headers with required alg, kid, and typ fields.

Before/After

// Before: hardcoded values
let iss_base64_details = ZkLoginClaim {
    value: "wiaXNzIjoiaHR0cHM6Ly9pZC50d2l0Y2gudHYvb2F1dGgyIiw".to_owned(),
    index_mod_4: 2,
};
let header_base64 = "eyJhbGciOiJSUzI1NiIsInR5cCI6IkpXVCIsImtpZCI6IjEifQ".to_owned();

// After: dynamic generation
let iss_claim_strategy = ("[a-z]{10,50}").prop_map(|iss_value| {
    let extended_claim = format!(r#""iss":"https://{}.example.com","#, iss_value);
    let value = Base64UrlUnpadded::encode_string(extended_claim.as_bytes());
    ZkLoginClaim { value, index_mod_4: 0 }
});

All 367 tests pass with the new implementation.

Original prompt

Work on TODO: write this implementation (from crates/iota-sdk-types/src/lib.rs)

Created from VS Code via the GitHub Pull Request extension.

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