test: add connection leak and pragma tests for db_pool.py

Add two new test classes to verify:
- ConnectionPool does not leak connections (fresh conn after close,
  context manager cleanup, thread isolation, repeated open/close cycles)
- SQLite pragmas (WAL, busy_timeout, cache_size) are correctly applied
  and scoped per-connection as used throughout the codebase

Fixes #944

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/infrastructure/test_db_pool.py

@@ -242,6 +242,145 @@ class TestCloseAll:
             conn.execute("SELECT 1")
 
 
+class TestConnectionLeaks:
+    """Test that connections do not leak."""
+
+    def test_get_connection_after_close_returns_fresh_connection(self, tmp_path):
+        """After close, get_connection() returns a new working connection."""
+        pool = ConnectionPool(tmp_path / "test.db")
+        conn1 = pool.get_connection()
+        pool.close_connection()
+
+        conn2 = pool.get_connection()
+        assert conn2 is not conn1
+        # New connection must be usable
+        cursor = conn2.execute("SELECT 1")
+        assert cursor.fetchone()[0] == 1
+        pool.close_connection()
+
+    def test_context_manager_does_not_leak_connection(self, tmp_path):
+        """After context manager exit, thread-local conn is cleared."""
+        pool = ConnectionPool(tmp_path / "test.db")
+        with pool.connection():
+            pass
+        # Thread-local should be cleaned up
+        assert pool._local.conn is None
+
+    def test_context_manager_exception_does_not_leak_connection(self, tmp_path):
+        """Connection is cleaned up even when an exception occurs."""
+        pool = ConnectionPool(tmp_path / "test.db")
+        try:
+            with pool.connection():
+                raise RuntimeError("boom")
+        except RuntimeError:
+            pass
+        assert pool._local.conn is None
+
+    def test_threads_do_not_leak_into_each_other(self, tmp_path):
+        """A connection opened in one thread is invisible to another."""
+        pool = ConnectionPool(tmp_path / "test.db")
+        # Open a connection on main thread
+        pool.get_connection()
+
+        visible_from_other_thread = []
+
+        def check():
+            has_conn = hasattr(pool._local, "conn") and pool._local.conn is not None
+            visible_from_other_thread.append(has_conn)
+
+        t = threading.Thread(target=check)
+        t.start()
+        t.join()
+
+        assert visible_from_other_thread == [False]
+        pool.close_connection()
+
+    def test_repeated_open_close_cycles(self, tmp_path):
+        """Repeated open/close cycles do not accumulate leaked connections."""
+        pool = ConnectionPool(tmp_path / "test.db")
+        for _ in range(50):
+            with pool.connection() as conn:
+                conn.execute("SELECT 1")
+            # After each cycle, connection should be cleaned up
+            assert pool._local.conn is None
+
+
+class TestPragmaApplication:
+    """Test that SQLite pragmas can be applied and persist on pooled connections.
+
+    The codebase uses WAL journal mode and busy_timeout pragmas on connections
+    obtained from the pool. These tests verify that pattern works correctly.
+    """
+
+    def test_wal_journal_mode_persists(self, tmp_path):
+        """WAL journal mode set on a pooled connection persists for its lifetime."""
+        pool = ConnectionPool(tmp_path / "test.db")
+        conn = pool.get_connection()
+        conn.execute("PRAGMA journal_mode=WAL")
+        mode = conn.execute("PRAGMA journal_mode").fetchone()[0]
+        assert mode == "wal"
+
+        # Same connection should retain the pragma
+        same_conn = pool.get_connection()
+        mode2 = same_conn.execute("PRAGMA journal_mode").fetchone()[0]
+        assert mode2 == "wal"
+        pool.close_connection()
+
+    def test_busy_timeout_persists(self, tmp_path):
+        """busy_timeout pragma set on a pooled connection persists."""
+        pool = ConnectionPool(tmp_path / "test.db")
+        conn = pool.get_connection()
+        conn.execute("PRAGMA busy_timeout=5000")
+        timeout = conn.execute("PRAGMA busy_timeout").fetchone()[0]
+        assert timeout == 5000
+        pool.close_connection()
+
+    def test_pragmas_apply_per_connection(self, tmp_path):
+        """Pragmas set on one thread's connection are independent of another's."""
+        pool = ConnectionPool(tmp_path / "test.db")
+        conn_main = pool.get_connection()
+        conn_main.execute("PRAGMA cache_size=9999")
+
+        other_cache = []
+
+        def check_pragma():
+            conn = pool.get_connection()
+            # Don't set cache_size — should get the default, not 9999
+            val = conn.execute("PRAGMA cache_size").fetchone()[0]
+            other_cache.append(val)
+            pool.close_connection()
+
+        t = threading.Thread(target=check_pragma)
+        t.start()
+        t.join()
+
+        # Other thread's connection should NOT have our custom cache_size
+        assert other_cache[0] != 9999
+        pool.close_connection()
+
+    def test_session_pragma_resets_on_new_connection(self, tmp_path):
+        """Session-level pragmas (cache_size) reset on a new connection."""
+        pool = ConnectionPool(tmp_path / "test.db")
+        conn1 = pool.get_connection()
+        conn1.execute("PRAGMA cache_size=9999")
+        assert conn1.execute("PRAGMA cache_size").fetchone()[0] == 9999
+        pool.close_connection()
+
+        conn2 = pool.get_connection()
+        cache = conn2.execute("PRAGMA cache_size").fetchone()[0]
+        # New connection gets default cache_size, not the previous value
+        assert cache != 9999
+        pool.close_connection()
+
+    def test_wal_mode_via_context_manager(self, tmp_path):
+        """WAL mode can be set within a context manager block."""
+        pool = ConnectionPool(tmp_path / "test.db")
+        with pool.connection() as conn:
+            conn.execute("PRAGMA journal_mode=WAL")
+            mode = conn.execute("PRAGMA journal_mode").fetchone()[0]
+            assert mode == "wal"
+
+
 class TestIntegration:
     """Integration tests for real-world usage patterns."""