feat: expose externalToken on AuthenticationResponse

[itaihanski]

Related Issues

https://github.com/descope/etc/issues/16830

Description

• Populate AuthenticationResponse.externalToken: String? from the server auth response
• Parsed on JwtServerResponse and passed through convert(), matching the Flutter and web SDKs

Must

• Tests
• Documentation

[shuni-bot]

🐕 Review complete — View session on Shuni Portal 🐾

[shuni-bot]

🐕 Suggested Reviewers

Selected the contributor who has contributed extensively across the relevant files to ensure broad expertise and familiarity with the code areas impacted by this PR.

Reviewer	Reason
shilgapira	ShilgaPira reviewed multiple files, including Responses.kt and TestUtils.kt, covering both main and test code relevant to the change.

_{Suggested by Shuni based on git history and PR context. Names are not @-mentioned to avoid notifying anyone — request a review from whoever fits best.}

[shuni-bot]

🐕 Shuni's Review

Exposes AuthenticationResponse.externalToken populated from the server auth response, threaded cleanly through JwtServerResponse → convert() → the public type.

No issues found — good bones! Parsing uses stringOrEmptyAsNull like sibling fields, the new nullable is defaulted, and both present/absent cases have tests. Woof!