test: improve test coverage (#290)

# 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%

Signed-off-by: Shingo OKAWA <[email protected]>

Author: ognis1205