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_context.rb

@@ -43,6 +43,42 @@ module MCP
       end
     end
 
+    # Delegates to the session so the request is scoped to the originating client.
+    # Falls back to `@context` (via `method_missing`) when `@notification_target`
+    # does not support elicitation.
+    def create_form_elicitation(**kwargs)
+      if @notification_target.respond_to?(:create_form_elicitation)
+        @notification_target.create_form_elicitation(**kwargs, related_request_id: @related_request_id)
+      elsif @context.respond_to?(:create_form_elicitation)
+        @context.create_form_elicitation(**kwargs, related_request_id: @related_request_id)
+      else
+        raise NoMethodError, "undefined method 'create_form_elicitation' for #{self}"
+      end
+    end
+
+    # Delegates to the session so the request is scoped to the originating client.
+    # Falls back to `@context` when `@notification_target` does not support URL mode elicitation.
+    def create_url_elicitation(**kwargs)
+      if @notification_target.respond_to?(:create_url_elicitation)
+        @notification_target.create_url_elicitation(**kwargs, related_request_id: @related_request_id)
+      elsif @context.respond_to?(:create_url_elicitation)
+        @context.create_url_elicitation(**kwargs, related_request_id: @related_request_id)
+      else
+        raise NoMethodError, "undefined method 'create_url_elicitation' for #{self}"
+      end
+    end
+
+    # Delegates to the session so the notification is scoped to the originating client.
+    def notify_elicitation_complete(**kwargs)
+      if @notification_target.respond_to?(:notify_elicitation_complete)
+        @notification_target.notify_elicitation_complete(**kwargs)
+      elsif @context.respond_to?(:notify_elicitation_complete)
+        @context.notify_elicitation_complete(**kwargs)
+      else
+        raise NoMethodError, "undefined method 'notify_elicitation_complete' for #{self}"
+      end
+    end
+
     def method_missing(name, ...)
       if @context.respond_to?(name)
         @context.public_send(name, ...)