feat: add auth preprocessor and update auth handler.

[kalenkevich]

Overview

This PR improves the test coverage and fixes inconsistencies in the authentication handling logic for the ADK-JS project. It addresses a bug in AuthHandler where exchange results were stored incorrectly, enhances existing tests to verify actual generated values (like URLs and states), and introduces a new unit test suite for the AuthPreprocessor.

Key Changes

Bug Fixes

AuthHandler

• Removed a type inconsistency in parseAndStoreAuthResponse where the full ExchangeResult object was unintentionally stored in session state. Now, it correctly stores only the nested AuthCredential (exchangedCredential.credential), aligning with how getAuthResponse retrieves it.

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Testing Enhancements

AuthHandler Tests

• New Coverage: Added comprehensive unit tests for parseAndStoreAuthResponse to cover:
  • Non-OAuth2 storage mechanics.
  • Early-return conditions for unsupported schemes.
  • Successful OAuth2 exchanges.
• Improved Assertions:
  • generateAuthUri tests now assert the actual generated URL string and query parameters (such as client_id, redirect_uri, scope, and dynamic state) rather than returning stubs.
  • generateAuthRequest tests assert usage of the generated credentials.

[NEW] AuthPreprocessor Tests

• Created unit tests covering AuthPreprocessor.runAsync generator logic:
  • Verification of early-return scenarios:
    • Agent is not an LlmAgent.
    • Active session has no events or user content.
    • Message history contains no adk_request_credential tool-resume responses.
  • Successful tool resumption scenario: verifies the preprocessor intercepts a user's credential input, updates state storage, parses resume target tool calls, and delegates continuity via handleFunctionCallsAsync.
• Standardized Vitest mocking conventions and TypeScript types for handleFunctionCallsAsync.