[auth] Add NonceUtil for JWT nonce extraction and id_token field to UberToken

[partha-uber]

Summary:

• Intent:

  • Provide a reusable utility for extracting the `nonce` claim from a JWT `id_token` payload without signature verification, matching the approach in the iOS SDK.
  • Add the `id_token` field to `UberToken` so PKCE flows can receive and inspect it.

• Changes:

  • New `NonceUtil.extractNonceFromIdToken()` — decodes the base64url payload segment and returns the `nonce` claim; returns null on malformed input.
  • Added `idToken` field to `UberToken`.
  • `NonceUtilTest` covers nonce-present, nonce-absent, malformed JWT, two-segment token, and invalid base64 cases.

🤖 Generated with Claude Code

Co-Authored-By: Claude [email protected]