Add CI probe for PDO-churn state buildup vs the 3 failing tests

Open N PDO connections in sequence (with INSERT/SELECT/CREATE TEMP
on each, then close) to simulate the suite's churn. After N closes,
run a fresh PDO with both failing-test query shapes (correlated
filter + temp-table create/drop) and report what each returns.

Vary N over {0, 1, 5, 20, 50, 100, 200, 500}. If the bugs manifest
past some threshold, we have a minimal repro and can bisect what
process-global state accumulates.

Author: JanJakes

.github/workflows/phpunit-tests-turso.yml

@@ -1007,6 +1007,63 @@ jobs:
               unset($pdo);
           }
 
+      - name: Probe Turso PDO churn — does state accumulate that breaks later queries?
+        continue-on-error: true
+        env:
+          LD_PRELOAD: ${{ steps.preload.outputs.value }}
+        run: |
+          php <<'PHP'
+          <?php
+          // The 3 failing tests all pass in fresh-PDO probes but fail
+          // mid-suite. This probe simulates the suite's PDO churn:
+          // open N PDOs, run a smattering of operations on each, close.
+          // Then in a final fresh PDO, run the failing-test query shape
+          // and see whether the bug manifests.
+          //
+          // If the bug reproduces past some N, we have a minimal repro
+          // and can bisect what specifically accumulates.
+          function setup_and_run(int $churn_count): array {
+              for ($i = 0; $i < $churn_count; $i++) {
+                  $p = new PDO('sqlite::memory:');
+                  $p->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
+                  $p->exec('CREATE TABLE t (id INT, v TEXT)');
+                  $p->exec("INSERT INTO t VALUES (1, 'a'), (2, 'b'), (3, 'c')");
+                  $p->query('SELECT * FROM t');
+                  // CREATE TEMP TABLE on each — exercises temp pager
+                  $p->exec('CREATE TEMPORARY TABLE t2 (id INT)');
+                  $p->exec("INSERT INTO t2 VALUES (1)");
+                  unset($p);
+              }
+              // Now in a fresh PDO, run the failing-test shape.
+              $p = new PDO('sqlite::memory:');
+              $p->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
+              $p->exec("CREATE TABLE _ist (table_schema TEXT, table_name TEXT)");
+              $p->exec("CREATE TABLE _isc (table_schema TEXT, table_name TEXT, column_name TEXT, extra TEXT)");
+              $p->exec("INSERT INTO _ist VALUES ('main','low'), ('main','high'), ('main','plain')");
+              $p->exec("INSERT INTO _isc VALUES ('main','low','id','auto_increment'), ('main','high','id','auto_increment'), ('main','plain','id','')");
+              $p->exec("CREATE TABLE sqlseq (name TEXT, seq INT)");
+              $p->exec("INSERT INTO sqlseq VALUES ('low',1), ('high',5)");
+              $derived = "(SELECT table_name AS NAME,
+                  (SELECT COALESCE(s.seq + 1, 1) FROM _isc AS c LEFT JOIN sqlseq AS s ON s.name = c.table_name
+                   WHERE c.extra = 'auto_increment' AND c.table_schema = t.table_schema AND c.table_name = t.table_name) AS AI
+                  FROM _ist AS t)";
+              $rows1 = $p->query("SELECT NAME FROM $derived WHERE AI > 3")->fetchAll(PDO::FETCH_COLUMN);
+              // also run a temp-table sequence
+              $temp_ok = true;
+              $temp_err = null;
+              try {
+                  $p->exec('CREATE TEMPORARY TABLE _ttest (id INTEGER PRIMARY KEY AUTOINCREMENT, v TEXT NOT NULL DEFAULT \'\')');
+                  $p->exec('DROP TABLE _ttest');
+              } catch (Throwable $e) { $temp_ok = false; $temp_err = $e->getMessage(); }
+              return ['filter_rows' => $rows1, 'temp_ok' => $temp_ok, 'temp_err' => $temp_err];
+          }
+          foreach ([0, 1, 5, 20, 50, 100, 200, 500] as $n) {
+              $r = setup_and_run($n);
+              $rows = json_encode($r['filter_rows']);
+              $temp = $r['temp_ok'] ? 'temp OK' : 'temp FAIL: '.$r['temp_err'];
+              echo "churn=$n  filter_rows=$rows  $temp\n";
+          }
+
       - name: Probe Turso correlated subquery in derived-table SELECT list
         continue-on-error: true
         env: