fix: address root cause of test hanging - improper ZMQ/C++ resource cleanup

Fixed the actual root cause instead of just masking it in tests:

1. Root Problem:
   - C++ side's ZmqDistanceComputer creates ZMQ connections but doesn't clean them
   - Python 3.9/3.13 are more sensitive to cleanup timing during shutdown

2. Core Fixes in SearcherBase and LeannSearcher:
   - Added cleanup() method to BaseSearcher that cleans ZMQ and embedding server
   - LeannSearcher.cleanup() now also handles ZMQ context cleanup
   - Both HNSW and DiskANN searchers now properly delete C++ index objects

3. Backend-Specific Cleanup:
   - HNSWSearcher.cleanup(): Deletes self.index to trigger C++ destructors
   - DiskannSearcher.cleanup(): Deletes self._index and resets state
   - Both force garbage collection after deletion

4. Test Infrastructure:
   - Added auto_cleanup_searcher fixture for explicit resource management
   - Global cleanup now more aggressive with ZMQ context destruction

This is the proper fix - cleaning up resources at the source, not just
working around the issue in tests. The hanging was caused by C++ side
ZMQ connections not being properly terminated when is_recompute=True.

tests/conftest.py

@@ -18,13 +18,37 @@ def global_test_cleanup() -> Generator:
     yield
 
     # Cleanup after all tests
+    print("\n🧹 Running global test cleanup...")
+
+    # 1. Force cleanup of any LeannSearcher instances
+    try:
+        import gc
+
+        # Force garbage collection to trigger __del__ methods
+        gc.collect()
+        time.sleep(0.2)
+    except Exception:
+        pass
+
+    # 2. Terminate ZMQ contexts more aggressively
     try:
         import zmq
 
-        # Set a very short linger on any remaining contexts
-        # This prevents blocking on context termination
+        # Get the global instance and destroy it
         ctx = zmq.Context.instance()
         ctx.linger = 0
+
+        # Force termination - this is aggressive but needed for CI
+        try:
+            ctx.destroy(linger=0)
+        except Exception:
+            pass
+
+        # Also try to terminate the default context
+        try:
+            zmq.Context.term(zmq.Context.instance())
+        except Exception:
+            pass
     except Exception:
         pass
 
@@ -78,6 +102,32 @@ def global_test_cleanup() -> Generator:
         pass
 
 
+@pytest.fixture
+def auto_cleanup_searcher():
+    """Fixture that automatically cleans up LeannSearcher instances."""
+    searchers = []
+
+    def register(searcher):
+        """Register a searcher for cleanup."""
+        searchers.append(searcher)
+        return searcher
+
+    yield register
+
+    # Cleanup all registered searchers
+    for searcher in searchers:
+        try:
+            searcher.cleanup()
+        except Exception:
+            pass
+
+    # Force garbage collection
+    import gc
+
+    gc.collect()
+    time.sleep(0.1)
+
+
 @pytest.fixture(autouse=True)
 def cleanup_after_each_test():
     """Cleanup after each test to prevent resource leaks."""