fix: Handle closed DuckDB connections gracefully in pool cleanup

When a query fails and closes the connection, the pool cleanup was
trying to execute SELECT 1 on the closed connection, causing
"Connection already closed!" errors on subsequent queries.

Now if a connection is invalid during cleanup, we:
1. Close it properly
2. Create a fresh connection
3. Return the fresh connection to the pool

This ensures the pool always contains valid connections and prevents
cascading connection errors.

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

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

Author: Ignacio Van Droogenbroeck

api/duckdb_pool_simple.py

@@ -145,8 +145,8 @@ def get_connection(self, timeout: float = 5.0):
             # CRITICAL: Always return and clean connection
             if conn is not None:
                 try:
-                    # Reset connection state (clears DuckDB internal caches)
-                    # Use a simple query to clear any cached results
+                    # Test if connection is still valid
+                    # If the connection is closed, this will raise an exception
                     result = conn.execute("SELECT 1").fetchall()
                     del result
 
@@ -158,13 +158,21 @@ def get_connection(self, timeout: float = 5.0):
                     if collected > 100:
                         logger.debug(f"Connection cleanup: collected {collected} objects")
 
+                    # Connection is valid, return to pool
+                    self.pool.put(conn)
+
                 except Exception as e:
-                    # If cleanup fails, log but don't crash
-                    logger.debug(f"Connection cleanup warning: {e}")
+                    # Connection is invalid/closed - create a fresh one
+                    logger.warning(f"Connection invalid during cleanup ({e}), creating fresh connection")
+                    try:
+                        # Close the bad connection properly
+                        conn.close()
+                    except Exception:
+                        pass
 
-                finally:
-                    # Always return connection to pool
-                    self.pool.put(conn)
+                    # Create and return a fresh connection to the pool
+                    fresh_conn = duckdb.connect(self.db_path)
+                    self.pool.put(fresh_conn)
 
     def get_metrics(self) -> Dict[str, Any]:
         """