feat: add structured capability types

Replace generic capability dictionaries with structured types for prompts,
resources, tools, and roots. This improves type safety and makes capability
features like listChanged and subscribe more explicit in the protocol.

mcp_python/client/session.py

@@ -26,6 +26,7 @@ from mcp_python.types import (
     PromptReference,
     ReadResourceResult,
     ResourceReference,
+    RootsCapability,
     ServerNotification,
     ServerRequest,
 )
@@ -69,12 +70,12 @@ class ClientSession(
                         capabilities=ClientCapabilities(
                             sampling=None,
                             experimental=None,
-                            roots={
+                            roots=RootsCapability(
                                 # TODO: Should this be based on whether we
                                 # _will_ send notifications, or only whether
                                 # they're supported?
-                                "listChanged": True
-                            },
+                                listChanged=True
+                            ),
                         ),
                         clientInfo=Implementation(name="mcp_python", version="0.1.0"),
                     ),