expose jag api functions in zts java client

[havetisyan]

Description

part of #3100 support. expose the jag token support in java client

Contribution Checklist:

• The pull request does not introduce any breaking changes
• I have read the contribution guidelines.
• Create an issue and link to the pull request.

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[gemini-code-assist]

Summary of Changes

Hello @havetisyan, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the ZTS Java client by introducing robust support for JWT Authorization Grant (JAG) API functions. It refactors the token request building process into a dedicated OAuthTokenRequestBuilder and updates the ZTSClient to provide new methods for obtaining and exchanging JAG tokens. Additionally, it includes critical null checks in RDL-generated client code for improved stability and expands the test suite to cover the new and modified functionalities.

Highlights

• New OAuth Token Request Builder: Introduced a new OAuthTokenRequestBuilder class in the ZTS Java client to provide a flexible and comprehensive way to construct OAuth token requests, supporting various grant types and parameters.
• JAG API Exposure: The ZTSClient now exposes new public methods, getJAGToken and getJAGExchangeToken, allowing clients to request and exchange JWT Authorization Grant (JAG) tokens.
• Refactored Token Retrieval Logic: The internal token retrieval logic in ZTSClient has been refactored into a new getToken method, which centralizes caching, prefetching, and error handling for all token types, and the old AccessTokenRequestBuilder has been removed.
• Robust Header Handling in RDL Clients: Added null checks for ETag and Location headers in several RDL-generated client files (MSDRDLGeneratedClient.java, ZMSRDLGeneratedClient.java, ZTSRDLGeneratedClient.java) to prevent potential NullPointerExceptions.
• Enhanced Test Coverage: New test files have been added for ClientResourceException, IdTokenCacheEntry, OAuthTokenRequestBuilder, ZTSClientCache, and ZTSClientNotification, significantly improving the test coverage for the ZTS Java client.

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[gemini-code-assist]

Code Review

This pull request introduces support for JWT Authorization Grant (JAG) tokens by adding a new OAuthTokenRequestBuilder and refactoring the ZTSClient to use it. This is a significant and well-structured change that improves the client's capabilities. Additionally, it includes important bug fixes to prevent potential NullPointerExceptions in generated client code when handling HTTP headers. My review focuses on a likely copy-paste error in the new token methods, a potential issue with cache key generation, and a point of clarification in the prefetch logic. The inclusion of comprehensive new tests is a great addition.