fix: resolve pre-commit format and coverage issues

- Collapse super().__init__() to single line for ruff format
- Add blank line after CursorCycleError class
- Simplify cursor cycle test to eliminate uncovered branch
- Add pragma: no branch for coverage.py false positive on nested async

Fixes CI failures on PR #3156

Signed-off-by: dongjiang <dongjiang1989@126.com>

Author: dongjiang1989

src/mcp/client/client.py

@@ -96,10 +96,7 @@ class PaginationExceededError(RuntimeError):
     """
 
     def __init__(self, method: str, max_pages: int) -> None:
-        super().__init__(
-            f"{method}: exceeded list_max_pages ({max_pages});"
-            " server pagination did not terminate"
-        )
+        super().__init__(f"{method}: exceeded list_max_pages ({max_pages}); server pagination did not terminate")
         self.method = method
         self.max_pages = max_pages
 
@@ -116,6 +113,7 @@ def __init__(self, method: str, cursor: str) -> None:
         self.method = method
         self.cursor = cursor
 
+
 _Connector = Callable[[AsyncExitStack, ConnectMode, bool], Awaitable["Dispatcher[Any]"]]
 """Resolved at ``__post_init__`` from the shape of ``server`` alone: enter whatever resources
 are needed onto the exit stack and hand back the ``Dispatcher`` ``ClientSession`` will drive.

tests/client/test_list_all_pagination.py

@@ -151,15 +151,14 @@ def dummy() -> str:  # pragma: no cover
 
     async with Client(server, mode="legacy", list_max_pages=0) as client:
         original = client.list_tools
-        cursors = ["cursorA", "cursorB", "cursorA"]  # cycle on 3rd call
+        cursors_seq = ["cursorA", "cursorB", "cursorA", "cursorA", "cursorA"]
         call_count = 0
 
         async def cycling_list_tools(*, cursor: str | None = None, **kwargs: object) -> ListToolsResult:
             nonlocal call_count
-            result = await original(cursor=cursor, **kwargs)
-            if call_count < len(cursors):
-                result.next_cursor = cursors[call_count]
             call_count += 1
+            result = await original(cursor=cursor, **kwargs)
+            result.next_cursor = cursors_seq[call_count - 1]  # pragma: no branch
             return result
 
         client.list_tools = cycling_list_tools  # type: ignore[method-assign]