Add long running tests of memory leak for Python SDK

Use the tools like https://github.com/4paradigm/OpenMLDB/blob/main/java/openmldb-jmh/src/main/java/com/_4paradigm/sql/stability/FEQLFZPerf.java and `valgrind`.

Look at the referenced Java JMH test in openmldb-jmh/src/main/java/com/_4paradigm/sql/stability/FEQLFZPerf.java to understand the pattern for long-running performance tests. Investigate the Python SDK structure to identify entry points for memory-intensive operations. Use valgrind or similar memory profiling tools to design tests that can run for extended periods and detect leaks. The goal is to create a test suite that can be integrated into the project's CI for ongoing memory leak detection.

Written by the indexing model from the issue text.