fix: added Missing test cases for cleanup of the databases and updated the readme.md file

[Yashh56]

This pull request improves the documentation and reliability of the database service and store tests. It adds more comprehensive test coverage for database deletion, strengthens test cleanup logic to avoid leftover test databases, and updates the documentation to clarify test setup and supported databases.

Documentation improvements:

• Updated README.md to reflect support for MariaDB, clarified navigation links, and added detailed instructions for running integration tests with Docker and environment variables. Also included a summary of available test suites and cleanup steps. [1] [2]

Testing reliability and coverage:

• Enhanced cleanup logic in databaseService.test.ts to ensure all test databases are deleted after tests, using both tracked IDs and a safety-net name filter.
• Added new test cases to databaseService.test.ts for successful deletion of a database and for error handling when attempting to delete a non-existent database.
• Increased cache expiration buffer and TTL in dbStore.test.ts to reduce flakiness due to timing issues. [1] [2]

General test improvements:

• Removed redundant instantiations of DatabaseService in each test case, streamlining the test code. [1] [2] [3] [4] [5] [6] [7] [8] [9]