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.

test/json_rpc_handler_test.rb

@@ -488,6 +488,43 @@ describe JsonRpcHandler do
       }
     end
 
+    it "returns a custom error code when RequestHandlerError has error_code set" do
+      register("test_method") do
+        raise MCP::Server::RequestHandlerError.new(
+          "Custom error",
+          nil,
+          error_code: -32042,
+          error_data: { elicitations: [{ id: "abc" }] },
+        )
+      end
+
+      handle jsonrpc: "2.0", id: 1, method: "test_method"
+
+      assert_rpc_error expected_error: {
+        code: -32042,
+        message: "Custom error",
+        data: { elicitations: [{ id: "abc" }] },
+      }
+    end
+
+    it "falls back to error_type when error_code is nil" do
+      register("test_method") do
+        raise MCP::Server::RequestHandlerError.new(
+          "Invalid params error",
+          nil,
+          error_type: :invalid_params,
+        )
+      end
+
+      handle jsonrpc: "2.0", id: 1, method: "test_method"
+
+      assert_rpc_error expected_error: {
+        code: -32602,
+        message: "Invalid params",
+        data: "Invalid params error",
+      }
+    end
+
     # 6 Batch
     #
     # To send several Request objects at the same time, the Client MAY send an Array filled with Request objects.