test: Close commissioner session to properly cleanup resources

Add explicit CloseSession calls in TC_CGEN_2_8 and TC_CGEN_2_9 to ensure
proper cleanup of commissioner library resources. This fixes an issue where
subsequent commissioner sessions could not be established due to lingering
resources from previous test runs.

Bug: Commissioner session establishment failures in subsequent test runs
Fix: Add explicit session cleanup after commissioning operations

Testing:
- Set PICS_USER_PROMPT=1 for manual intervention testing
- Verified with terms-and-conditions-app test harness
- Confirmed proper cleanup by:
  1. Running initial commissioning
  2. Performing factory reset (process kill + KVS cleanup)
  3. Successfully re-establishing commissioner session
- Test passes with manual intervention at factory reset and commissioning prompts

src/python_testing/TC_CGEN_2_8.py

@@ -119,6 +119,9 @@ async def test_TC_CGEN_2_8(self):
             "First CommissioningComplete failed",
         )
 
+        # Close the commissioner session with the device to clean up resources
+        commissioner.CloseSession(nodeid=self.dut_node_id)
+
         # Step 5: Factory reset is handled by test operator
         self.step(5)
         if not self.check_pics('PICS_USER_PROMPT'):

src/python_testing/TC_CGEN_2_9.py

@@ -147,6 +147,9 @@ async def test_TC_CGEN_2_9(self):
         self.step(5)
         await self.remove_commissioner_fabric()
 
+        # Close the commissioner session with the device to clean up resources
+        commissioner.CloseSession(nodeid=self.dut_node_id)
+
         # Step 6: Put device in commissioning mode (requiring user input, so skip in CI)
         self.step(6)
         if not self.check_pics('PICS_USER_PROMPT'):