fix: add JSON tags to ElicitationCapabilities fields

The Form and URL fields in ElicitationCapabilities were missing JSON
struct tags, causing them to serialize as "Form" and "URL" (capitalized)
instead of the lowercase "form" and "url" required by the MCP protocol.

This caused initialization failures when clients advertised elicitation
capabilities, as servers would not recognize the incorrectly-cased keys.

Discovered via downstream issue: https://github.com/docker/cagent/issues/1574

mcp/protocol.go

@@ -988,8 +988,8 @@ type SamplingCapabilities struct{}
 //
 // If neither Form nor URL is set, the 'Form' capabilitiy is assumed.
 type ElicitationCapabilities struct {
-	Form *FormElicitationCapabilities
-	URL  *URLElicitationCapabilities
+	Form *FormElicitationCapabilities `json:"form,omitempty"`
+	URL  *URLElicitationCapabilities  `json:"url,omitempty"`
 }
 
 // FormElicitationCapabilities describes capabilities for form elicitation.