fix: move cancel_scope.cancel() inside ClientSession for 3.11 coverage

The tg.cancel_scope.cancel() calls placed after the 'async with ClientSession'
block were not being tracked by coverage on Python 3.11, causing 5 statement
misses and dropping test file coverage below 100%.

Moving the cancel calls inside the ClientSession context manager ensures they
execute while coverage tracking is still active, achieving 100% statement and
branch coverage on all Python versions.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: g97iulio1609

tests/client/test_list_changed_callbacks.py

@@ -70,7 +70,7 @@ async def run_server():
             with anyio.fail_after(2):
                 await callback_called.wait()
 
-        tg.cancel_scope.cancel()
+            tg.cancel_scope.cancel()
 
 
 async def test_prompt_list_changed_callback():
@@ -123,7 +123,7 @@ async def run_server():
             with anyio.fail_after(2):
                 await callback_called.wait()
 
-        tg.cancel_scope.cancel()
+            tg.cancel_scope.cancel()
 
 
 async def test_resource_list_changed_callback():
@@ -176,7 +176,7 @@ async def run_server():
             with anyio.fail_after(2):
                 await callback_called.wait()
 
-        tg.cancel_scope.cancel()
+            tg.cancel_scope.cancel()
 
 
 async def test_list_changed_default_no_error():
@@ -228,7 +228,7 @@ async def run_server():
             # Give the session a moment to process
             await anyio.sleep(0.1)
 
-        tg.cancel_scope.cancel()
+            tg.cancel_scope.cancel()
 
 
 async def test_callback_exception_does_not_crash_session():
@@ -288,4 +288,4 @@ async def run_server():
             # Session should still be alive — verify by waiting for processing
             await anyio.sleep(0.1)
 
-        tg.cancel_scope.cancel()
+            tg.cancel_scope.cancel()