mcp: add Capabilities fields to ServerOptions and ClientOptions (#713)

This allows users to configure default capabilities, disable defaults,
or suppress listChanged notifications by setting ListChanged: false.

Key changes:
- Add ServerOptions.Capabilities and ClientOptions.Capabilities fields.
- Merge user-provided capabilities with dynamically-added ones (tools,
  prompts, resources, sampling, elicitation).
- Check ListChanged before sending list-changed notifications.
- Deprecate HasPrompts, HasResources, HasTools on ServerOptions.
- Remove unreleased ElicitationModes field from ClientOptions.
- Add clone methods to capability structs using shallowClone helper.
- Add synctest-based tests for notification behavior (go1.25).
- Add TestClientCapabilitiesOverWire and TestServerCapabilitiesOverWire
- Document the new Capabilities API in docs/

Updates #706

mcp/error_test.go

@@ -213,7 +213,13 @@ func TestURLElicitationRequired(t *testing.T) {
 
 		// Create client with elicitation handler and middleware.
 		client := NewClient(testImpl, &ClientOptions{
-			ElicitationModes: []string{"url"},
+			Capabilities: &ClientCapabilities{
+				Roots:   RootCapabilities{ListChanged: true},
+				RootsV2: &RootCapabilities{ListChanged: true},
+				Elicitation: &ElicitationCapabilities{
+					URL: &URLElicitationCapabilities{},
+				},
+			},
 			ElicitationHandler: func(ctx context.Context, req *ElicitRequest) (*ElicitResult, error) {
 				elicitCalled = true
 				elicitURL = req.Params.URL