mcp: fix broken client root capabilities (#698)

To address #607, add ClientCapabilities.RootsV2, and populate it when
constructing sending and receiving InitializeParams.

Fixes #607

docs/rough_edges.md

@@ -30,3 +30,9 @@ v2.
 
 - `AudioContent.MarshalJSON` should have had a pointer receiver, to be
   consistent with other content types.
+
+- `ClientCapabilities.Roots` should have been a distinguished struct pointer
+  ([see #607](https://github.com/modelcontextprotocol/go-sdk/issues/607)).
+
+  **Workaround**: use `ClientCapabilities.RootsV2`, which aligns with the
+  semantics of other capability fields.

docs/troubleshooting.md

@@ -51,7 +51,7 @@ func ExampleLoggingTransport() {
 
 	// Output:
 	// read: {"jsonrpc":"2.0","id":1,"result":{"capabilities":{"logging":{}},"protocolVersion":"2025-06-18","serverInfo":{"name":"server","version":"v0.0.1"}}}
-	// write: {"jsonrpc":"2.0","id":1,"method":"initialize","params":{"capabilities":{"roots":{"listChanged":true}},"clientInfo":{"name":"client","version":"v0.0.1"},"protocolVersion":"2025-06-18"}}
+	// write: {"jsonrpc":"2.0","id":1,"method":"initialize","params":{"clientInfo":{"name":"client","version":"v0.0.1"},"protocolVersion":"2025-06-18","capabilities":{"roots":{"listChanged":true}}}}
 	// write: {"jsonrpc":"2.0","method":"notifications/initialized","params":{}}
 }
 ```