test: improve test coverage

[ognis1205]

Description

Refactored tests/client/test_auth_middleware.py and added test cases to improve coverage for credentials.py and interceptor.py.

• src/a2a/client/auth/credentials.py: 94% -> 100%
• src/a2a/client/auth/interceptor.py: 71% -> 94%

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Fixes N/A 🦕

[gemini-code-assist]

Summary of Changes

Hello @ognis1205, 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 test suite for the client's authentication middleware. It introduces new test cases and refactors existing ones to ensure robust coverage of credential handling and security scheme application, directly addressing the stated goal of improving code coverage for credentials.py (94% -> 100%) and interceptor.py (71% -> 98%).

Highlights

• Test Refactoring: Consolidated and streamlined authentication tests using helper functions and a parametrized test structure (AuthTestCase), significantly reducing code duplication and improving maintainability. This includes new helper functions like build_success_response, build_send_message_request, and send_message.
• Expanded Authentication Coverage: Added comprehensive test cases for AuthInterceptor to cover various security schemes (API Key, OAuth2, OIDC, Bearer) and edge cases, such as scenarios with no agent card, unsupported schemes, or missing scheme definitions.
• Improved Credential Store Testing: Enhanced the InMemoryContextCredentialStore tests to ensure correct storage, retrieval, and overwriting of credentials based on session IDs.

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[gemini-code-assist]

Code Review

The pull request improves test coverage and refactors the authentication middleware tests. There are a couple of areas for improvement: one is a minor readability enhancement in a test case definition, and the other is a more significant issue where a test doesn't correctly verify its intended scenario.

[ognis1205]

Thanks for the approval!

I believe this is ready to be merged. I’m planning to improve test coverage for the remaining modules incrementally, so it would be great if this PR could be merged to keep changes small and focused. Let me know if anything else is needed!