Support elicitation/create per MCP specification

## Motivation and Context

The MCP specification (2025-11-25) defines elicitation as a server-to-client
request mechanism that allows servers to request user input during tool execution:
https://modelcontextprotocol.io/specification/2025-11-25/client/elicitation

This aligns with the Python SDK, which also exposes separate methods for form and URL mode elicitation.
Three methods are added following the Python SDK's API design:

- `create_form_elicitation`: collects structured data via JSON Schema
- `create_url_elicitation`: directs users to external URLs for sensitive
  interactions (OAuth, credentials, etc.)
- `notify_elicitation_complete`: notifies the client that a URL mode
  interaction has completed

`URLElicitationRequiredError` (error code `-32042`) is defined as a subclass of `RequestHandlerError`
with its own `error_code` and `error_data` attributes. `JsonRpcHandler#handle_request_error` is
extended to check for these attributes so that MCP-specific error codes can be propagated without
embedding MCP knowledge in the generic JSON-RPC layer.

## How Has This Been Tested?

- Unit tests for `ServerSession` form/URL mode, capability checks, nil
  capabilities, notification, and `URLElicitationRequiredError`
- `ServerContext` delegation tests for all three elicitation methods
- `JsonRpcHandler` tests for custom error code and fallback behavior
- `Methods` capability test for `NOTIFICATIONS_ELICITATION_COMPLETE`
- Conformance tests pass (40/40), including `tools-call-elicitation`,
  `elicitation-sep1034-defaults`, and `elicitation-sep1330-enums`
- All tests pass (`rake test`), RuboCop is clean
- Verified form mode elicitation end-to-end with a sample MCP server
  running in Claude Code

## Breaking Changes

None. This is a new feature addition.

lib/mcp/server.rb

@@ -31,14 +31,27 @@ module MCP
     MAX_COMPLETION_VALUES = 100
 
     class RequestHandlerError < StandardError
-      attr_reader :error_type
-      attr_reader :original_error
+      attr_reader :error_type, :original_error, :error_code, :error_data
 
-      def initialize(message, request, error_type: :internal_error, original_error: nil)
+      def initialize(message, request, error_type: :internal_error, original_error: nil, error_code: nil, error_data: nil)
         super(message)
         @request = request
         @error_type = error_type
         @original_error = original_error
+        @error_code = error_code
+        @error_data = error_data
+      end
+    end
+
+    class URLElicitationRequiredError < RequestHandlerError
+      def initialize(elicitations)
+        super(
+          "URL elicitation required",
+          nil,
+          error_type: :url_elicitation_required,
+          error_code: -32042,
+          error_data: { elicitations: elicitations },
+        )
       end
     end
 
@@ -114,7 +127,6 @@ module MCP
         # No op handlers for currently unsupported methods
         Methods::RESOURCES_SUBSCRIBE => ->(_) { {} },
         Methods::RESOURCES_UNSUBSCRIBE => ->(_) { {} },
-        Methods::ELICITATION_CREATE => ->(_) {},
       }
       @transport = transport
     end