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/mcp/server_context_test.rb

@@ -83,6 +83,62 @@ module MCP
       assert_equal "Fallback", result[:content][:text]
     end
 
+    test "ServerContext#create_form_elicitation delegates to notification_target" do
+      notification_target = mock
+      notification_target.expects(:create_form_elicitation).with(
+        message: "Please provide your name",
+        requested_schema: { type: "object", properties: { name: { type: "string" } } },
+        related_request_id: nil,
+      ).returns(action: "accept", content: { name: "test_user" })
+
+      context = mock
+      progress = Progress.new(notification_target: notification_target, progress_token: nil)
+
+      server_context = ServerContext.new(context, progress: progress, notification_target: notification_target)
+
+      result = server_context.create_form_elicitation(
+        message: "Please provide your name",
+        requested_schema: { type: "object", properties: { name: { type: "string" } } },
+      )
+
+      assert_equal "accept", result[:action]
+    end
+
+    test "ServerContext#create_url_elicitation delegates to notification_target" do
+      notification_target = mock
+      notification_target.expects(:create_url_elicitation).with(
+        message: "Please authorize",
+        url: "https://example.com/oauth",
+        elicitation_id: "abc-123",
+        related_request_id: nil,
+      ).returns(action: "accept")
+
+      context = mock
+      progress = Progress.new(notification_target: notification_target, progress_token: nil)
+
+      server_context = ServerContext.new(context, progress: progress, notification_target: notification_target)
+
+      result = server_context.create_url_elicitation(
+        message: "Please authorize",
+        url: "https://example.com/oauth",
+        elicitation_id: "abc-123",
+      )
+
+      assert_equal "accept", result[:action]
+    end
+
+    test "ServerContext#notify_elicitation_complete delegates to notification_target" do
+      notification_target = mock
+      notification_target.expects(:notify_elicitation_complete).with(elicitation_id: "abc-123")
+
+      context = mock
+      progress = Progress.new(notification_target: notification_target, progress_token: nil)
+
+      server_context = ServerContext.new(context, progress: progress, notification_target: notification_target)
+
+      server_context.notify_elicitation_complete(elicitation_id: "abc-123")
+    end
+
     test "ServerContext delegates to custom object context" do
       context = Object.new
       def context.custom_method