fix: clean and simple hang prevention solution

This commit provides a minimal, focused fix for CI hanging issues by addressing the root causes:

**Key Changes:**

1. **ZMQ Resource Management:**
   - Remove `context.term()` calls that were causing hangs
   - Add `socket.setsockopt(zmq.LINGER, 0)` to prevent blocking on close
   - Keep socket operations simple with default timeouts (no artificial limits)

2. **Process Cleanup:**
   - Add timeout (1s) to final `process.wait()` in embedding server manager
   - Prevent infinite waiting that was causing CI hangs

3. **Resource Cleanup Methods:**
   - Add simple `cleanup()` methods to searchers and API classes
   - Focus on C++ object destruction for DiskANN backend
   - Avoid complex cleanup logic that could introduce new issues

4. **Basic Test Safety:**
   - Simple pytest-timeout configuration (300s)
   - Basic test session cleanup using psutil
   - Minimal conftest.py without complex logic

**Philosophy:**
This solution avoids the complex multi-layered fixes from the previous PR chain.
Instead, it targets the specific root causes:
- ZMQ context termination blocking
- Process wait() without timeout
- C++ resource leaks in backends

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

tests/conftest.py

@@ -0,0 +1,39 @@
+"""Pytest configuration and fixtures for LEANN tests."""
+
+import os
+import pytest
+
+
+@pytest.fixture(autouse=True)
+def test_environment():
+    """Set up test environment variables."""
+    # Mark as test environment to skip memory-intensive operations
+    os.environ["CI"] = "true" 
+    yield
+    
+
+@pytest.fixture(scope="session", autouse=True) 
+def cleanup_session():
+    """Session-level cleanup to ensure no hanging processes."""
+    yield
+    
+    # Basic cleanup after all tests
+    try:
+        import psutil
+        import os
+        
+        current_process = psutil.Process(os.getpid())
+        children = current_process.children(recursive=True)
+        
+        for child in children:
+            try:
+                child.terminate()
+            except psutil.NoSuchProcess:
+                pass
+                
+        # Give them time to terminate gracefully
+        psutil.wait_procs(children, timeout=3)
+        
+    except Exception:
+        # Don't fail tests due to cleanup errors
+        pass