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_session.rb

@@ -47,6 +47,35 @@ module MCP
       send_to_transport_request(Methods::SAMPLING_CREATE_MESSAGE, params, related_request_id: related_request_id)
     end
 
+    # Sends an `elicitation/create` request (form mode) scoped to this session.
+    def create_form_elicitation(message:, requested_schema:, related_request_id: nil)
+      unless client_capabilities&.dig(:elicitation)
+        raise "Client does not support elicitation. " \
+          "The client must declare the `elicitation` capability during initialization."
+      end
+
+      params = { mode: "form", message: message, requestedSchema: requested_schema }
+      send_to_transport_request(Methods::ELICITATION_CREATE, params, related_request_id: related_request_id)
+    end
+
+    # Sends an `elicitation/create` request (URL mode) scoped to this session.
+    def create_url_elicitation(message:, url:, elicitation_id:, related_request_id: nil)
+      unless client_capabilities&.dig(:elicitation, :url)
+        raise "Client does not support URL mode elicitation. " \
+          "The client must declare the `elicitation.url` capability during initialization."
+      end
+
+      params = { mode: "url", message: message, url: url, elicitationId: elicitation_id }
+      send_to_transport_request(Methods::ELICITATION_CREATE, params, related_request_id: related_request_id)
+    end
+
+    # Sends an elicitation complete notification scoped to this session.
+    def notify_elicitation_complete(elicitation_id:)
+      send_to_transport(Methods::NOTIFICATIONS_ELICITATION_COMPLETE, { elicitationId: elicitation_id })
+    rescue => e
+      @server.report_exception(e, notification: "elicitation_complete")
+    end
+
     # Sends a progress notification to this session only.
     def notify_progress(progress_token:, progress:, total: nil, message: nil, related_request_id: nil)
       params = {