Add MCP::Elicitation::EnumSchema builders for SEP-1330

[koic]

Motivation and Context

MCP 2025-11-25 (SEP-1330) replaces the ad-hoc enumNames convention with four standards-based enum schema variants for elicitation requests: titled/untitled single-select and titled/untitled multi-select. The TypeScript SDK ships typed Zod definitions for each shape (packages/core/src/types/schemas.ts), and the Python SDK relies on hand-built schemas with the same shapes in its conformance fixtures.

Adds builder class methods on MCP::Elicitation::EnumSchema that produce the canonical JSON-Schema shapes for each variant, plus a legacy_titled builder retained for backward compatibility with clients that have not yet migrated. Each constructor returns an EnumSchema instance whose to_h produces the property-value Hash to pass through requested_schema: on ServerSession#create_form_elicitation.

Each builder maps one-to-one to the spec's schema type names in schema/2025-11-25/schema.ts, which the TypeScript and Python SDKs also carry verbatim in their type layers:

Spec type (schema.ts)	Builder
UntitledSingleSelectEnumSchema	untitled_single_select
TitledSingleSelectEnumSchema	titled_single_select
UntitledMultiSelectEnumSchema	untitled_multi_select
TitledMultiSelectEnumSchema	titled_multi_select
LegacyTitledEnumSchema (deprecated in the spec)	legacy_titled
EnumSchema (the union of all five)	the MCP::Elicitation::EnumSchema class itself

These shapes stay relevant on the stateless 2026-07-28 lifecycle: elicitation is not deprecated by SEP-2577 and reaches modern clients as elicitation/create requests embedded in SEP-2322 input_required results, whose requestedSchema uses the same variants. The README documents that pairing; the example references MCP::Server::InputRequiredResult, so this change should merge after the SEP-2322 server-side PR.

The conformance server now builds the enum schemas of its test_elicitation_sep1330_enums tool (and the status enum of the SEP-1034 defaults tool) through these builders instead of hand-written hashes, so the conformance run exercises the exact schemas the SDK produces for users.

How Has This Been Tested?

test/mcp/elicitation/enum_schema_test.rb covers each of the five shapes with happy-path emission, default propagation, title / description propagation, and the four ArgumentError cases (empty values, missing :value, missing :title, mismatched value_titles length). Two additional regression tests assert that empty-but-non-nil defaults survive to_h:

• EnumSchema.untitled_single_select(default: "") keeps the empty string in the emitted Hash
• EnumSchema.untitled_multi_select(default: []) keeps the empty array in the emitted Hash

bundle exec rake (tests, RuboCop, and conformance baseline) passes; the elicitation-sep1330-enums and elicitation-sep1034-defaults conformance scenarios pass with the builder-produced schemas, confirming they emit the same wire shapes as the previous hand-written hashes.

Breaking Changes

None. The change is purely additive: new module MCP::Elicitation::EnumSchema with new class methods. Existing callers of ServerSession#create_form_elicitation that pass raw hashes continue to work unchanged.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed