sketch: refactor lowlevel server to handler pattern

[maxisbey]

Replaces decorator-based handler registration with RequestHandler/NotificationHandler classes.

Changes

• New files: request_handler.py and notification_handler.py with typed overloaded constructors
• Server dispatch: type(req) → req.method (string-based dispatch)
• Handler dicts: dict[type, Callable] → dict[str, RequestHandler] / dict[str, NotificationHandler]
• Registration: Decorator methods → constructor handlers list + add_handler() method
• Context: RequestContext built and passed directly to handlers; request_ctx contextvar removed
• Removed: All 12 decorator methods, tool cache, _make_error_result, _get_cached_tool_definition, new_server.py sketch

Known breakages

• MCPServer._setup_handlers() — calls removed decorator methods
• ExperimentalHandlers — writes to handler dicts with type keys
• All lowlevel server tests — use decorator pattern
• All examples — use decorator pattern