test(c): add embedding and multi-connection tests, refactor memory checks

Introduces new test cases for embedding generation, context switching, sampler roundtrip, document ingestion, error recovery, and concurrent/multi-role connections. Refactors memory leak checks into a reusable helper and adds a --test argument to run a single test by name. Improves test coverage for extension lifecycle, error handling, and resource cleanup.

Author: andinux