Fix test assertions: change 'using notebook' to 'Successfully activate notebook'

Addresses feedback from @echarles and @ChengJiale150 on PR #138.

The three failing tests were checking for incorrect string in output:
- Expected: 'using notebook'
- Actual: 'Successfully activate notebook'

Fixed assertions in:
- test_rtc_mode_for_cell_operations (line 48)
- test_reading_tools_see_unsaved_changes (line 130)
- test_jupyter_collaboration_extension_loaded (line 207)

This matches the assertion pattern used in tests/test_tools.py.

🤖 Generated with Claude Code

Co-Authored-By: Claude <[email protected]>

Author: delphos-mike

tests/test_rtc_mode.py

@@ -45,7 +45,7 @@ async def test_rtc_mode_for_cell_operations(
             "test_rtc.ipynb",
             mode="create"
         )
-        assert "using notebook" in create_result
+        assert "Successfully activate notebook" in create_result
         logger.info("✓ Created test notebook")
 
         # 2. Insert a cell (should use RTC mode, not file mode)
@@ -127,7 +127,7 @@ async def test_reading_tools_see_unsaved_changes(
             "test_read_live.ipynb",
             mode="create"
         )
-        assert "using notebook" in create_result
+        assert "Successfully activate notebook" in create_result
 
         insert_result = await mcp_client_parametrized.insert_cell(
             0,
@@ -204,7 +204,7 @@ async def test_jupyter_collaboration_extension_loaded(
             "test_extension.ipynb",
             mode="create"
         )
-        assert "using notebook" in create_result
+        assert "Successfully activate notebook" in create_result
 
         # If we got here, the extension infrastructure is working
         logger.info("✓ jupyter-collaboration extension is functional")