Improve the entire specification test suite

[mridang]

Which Problems Are Solved

• SessionService endpoints lacked dedicated, end-to-end tests.
• Authentication tests were scattered, too tightly coupled to UserService flows, and inconsistently named.
• Test organization and documentation varied across languages.

How the Problems Are Solved

• Added full SessionService test suites covering create, retrieve, list, update and delete operations.
• Refactored all auth tests into an auth/ subdirectory and renamed them to a consistent Use…Spec pattern.
• Simplified auth tests to exercise the SettingsService’s general-settings endpoint rather than user-creation flows.
• Centralized environment-variable handling and client instantiation for cleaner, repeatable setup.

Additional Changes

• Unified docstrings and comments across all test suites as runnable examples.
• Standardized test structure, naming conventions, and error assertions.
• Introduced helper fixtures/utilities for base URLs, tokens, credentials and key files.

Additional Context

None.