Refactor: Use httptest for HTTP layer testing

[jcscottiii]

This PR improves the reliability and maintainability of our HTTP tests by transitioning from direct calls to the generated OpenAPI server methods to using the httptest package for true HTTP-level testing.

Motivation:

Currently, our tests bypass the HTTP layer, which limits their ability to identify issues with request handling, middleware, and serialization. This refactor provides a more accurate representation of real-world scenarios and enables better testing of components like the upcoming auth middleware.

Changes:

• httptest Integration: All HTTP handler tests now utilize httptest to simulate actual HTTP requests and responses. This involves updating test inputs to *http.Request and expected outputs to *http.Response.
• Centralized Assertions: Common assertion logic has been extracted into helper functions like assertTestServerRequest and assertMockCallCount to reduce code duplication and improve consistency across tests.
• OpenAPI Updates: Added TODO notes to deprecate outdated parameters in the OpenAPI document to maintain API consistency with other parameters.

Benefits:

• Increased Test Accuracy: Tests now accurately reflect the HTTP request lifecycle.
• Improved Code Maintainability: Centralized assertions reduce redundancy and simplify test updates.
• Enhanced Testability: Enables easier testing of middleware and other HTTP-related components.

Future Work:

• This refactor facilitates the implementation of the auth middleware in a subsequent PR. Needed for the saved searches work.