fix: add ORDER BY to combined SQL for deterministic result ordering

Individual SQL queries' ORDER BY clauses are not preserved when combined
with UNION ALL. Wrap the entire result in SELECT and ORDER BY cache_key!
to ensure consistent, deterministic ordering for tests.

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

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

Author: psteinroe

crates/pgls_splinter/src/lib.rs

@@ -109,7 +109,11 @@ pub async fn run_splinter(
             }
         })
         .collect();
-    let combined_sql = processed_queries.join("\n\nUNION ALL\n\n");
+    // Add ORDER BY to ensure deterministic ordering across all results
+    let combined_sql = format!(
+        "SELECT * FROM (\n{}\n) AS all_results ORDER BY \"cache_key!\"",
+        processed_queries.join("\n\nUNION ALL\n\n")
+    );
 
     // Execute the combined query
     let mut tx = params.conn.begin().await?;