Add client handling for sampling, list roots, ping (#218)

Adds sampling and list roots callbacks to the ClientSession, allowing the client to handle requests from the server.

Co-authored-by: TerminalMan <84923604+SecretiveShell@users.noreply.github.com>
Co-authored-by: David Soria Parra <davidsp@anthropic.com>

tests/client/test_list_roots_callback.py

@@ -0,0 +1,70 @@
+import pytest
+from pydantic import FileUrl
+
+from mcp.client.session import ClientSession
+from mcp.server.fastmcp.server import Context
+from mcp.shared.context import RequestContext
+from mcp.shared.memory import (
+    create_connected_server_and_client_session as create_session,
+)
+from mcp.types import (
+    ListRootsResult,
+    Root,
+    TextContent,
+)
+
+
+@pytest.mark.anyio
+async def test_list_roots_callback():
+    from mcp.server.fastmcp import FastMCP
+
+    server = FastMCP("test")
+
+    callback_return = ListRootsResult(
+        roots=[
+            Root(
+                uri=FileUrl("file://users/fake/test"),
+                name="Test Root 1",
+            ),
+            Root(
+                uri=FileUrl("file://users/fake/test/2"),
+                name="Test Root 2",
+            ),
+        ]
+    )
+
+    async def list_roots_callback(
+        context: RequestContext[ClientSession, None],
+    ) -> ListRootsResult:
+        return callback_return
+
+    @server.tool("test_list_roots")
+    async def test_list_roots(context: Context, message: str):
+        roots = await context.session.list_roots()
+        assert roots == callback_return
+        return True
+
+    # Test with list_roots callback
+    async with create_session(
+        server._mcp_server, list_roots_callback=list_roots_callback
+    ) as client_session:
+        # Make a request to trigger sampling callback
+        result = await client_session.call_tool(
+            "test_list_roots", {"message": "test message"}
+        )
+        assert result.isError is False
+        assert isinstance(result.content[0], TextContent)
+        assert result.content[0].text == "true"
+
+    # Test without list_roots callback
+    async with create_session(server._mcp_server) as client_session:
+        # Make a request to trigger sampling callback
+        result = await client_session.call_tool(
+            "test_list_roots", {"message": "test message"}
+        )
+        assert result.isError is True
+        assert isinstance(result.content[0], TextContent)
+        assert (
+            result.content[0].text
+            == "Error executing tool test_list_roots: List roots not supported"
+        )