feat: Add comprehensive unit tests and refactor for testability

[acramatte]

This commit introduces unit tests for major parts of the application, including authentication logic, chirp handlers, and login handlers.

Key changes:

• Added unit tests for internal/auth/auth.go, covering password hashing, JWT operations, and token/API key extraction.
• Added unit tests for handler_chirps.go, including chirp creation, retrieval, deletion, and body cleaning. This required:
  • Modifying handler_chirps.go to use chi.URLParam() for path parameters to enable easier testing.
• Added unit tests for handler_login.go, covering user login, token refresh, and token revocation.

To support testability, the following refactoring was performed:

• The db field in main.apiConfig was changed from a concrete *database.Queries type to a database.Querier interface. A Querier interface was defined in internal/database/db.go encompassing all necessary database methods.
• Key authentication functions (ValidateJWT, GetBearerToken, GetAPIKey) were made injectable by adding them as fields to main.apiConfig. Handler functions were updated to use these injectable functions.

Finally, README.md was updated with a "Running Tests" section, instructing you on how to execute the test suite using go test ./....

The tests for GetAPIKey and GetBearerToken in internal/auth/auth_test.go correctly fail for specific edge cases where the token/key string is empty after the prefix, as the underlying functions do not currently return errors for these inputs. These functions were not modified as part of this work.