Remove LegacyTitledEnumSchema API

[Copilot]

Breaking Change

Removes the deprecated LegacyTitledEnumSchema class from the elicitation API. The legacy schema used enumNames for titled enum options; migrate to TitledSingleSelectEnumSchema which uses oneOf with const/title per SEP-1330.

Changes

Protocol Changes

• Removed LegacyTitledEnumSchema class and related converter logic
• JSON with enumNames property now deserializes as UntitledSingleSelectEnumSchema (ignoring enumNames)
• Removed LegacyTitledEnumSchema from GetDefaultAsJsonElement switch expression

Diagnostics

• Removed obsoletions constants for MCP9001
• Added comment reserving MCP9001 to prevent ID reuse

Tests & Documentation

• Removed LegacyTitledEnumSchema tests and conformance server usage
• Updated elicitation.md and XML documentation

Migration

Before:

new ElicitRequestParams.LegacyTitledEnumSchema
{
    Enum = ["dev", "staging", "prod"],
    EnumNames = ["Development", "Staging", "Production"]
}

After:

new ElicitRequestParams.TitledSingleSelectEnumSchema
{
    OneOf = [
        new() { Const = "dev", Title = "Development" },
        new() { Const = "staging", Title = "Staging" },
        new() { Const = "prod", Title = "Production" }
    ]
}

Original prompt

Remove the [Obsolete] LegacyTitledEnumSchema API and label the PR that does this as a breaking-change.

Created from Copilot CLI via the copilot delegate command.

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[jeffhandley]

@mikekistler @stephentoub - Do you think it's appropriate to go ahead and remove this? We declared in https://modelcontextprotocol.github.io/csharp-sdk/versioning.html#obsolete-apis, "Within a MAJOR version update, obsolete APIs might be removed." It's our only obsolete API.

[stephentoub]

Do you think it's appropriate to go ahead and remove this

It's still in the spec:
https://github.com/modelcontextprotocol/modelcontextprotocol/blob/b84af972500d77e2f42b139c24ecaa8091aa3201/schema/2025-11-25/schema.ts#L2448

/**
 * Use TitledSingleSelectEnumSchema instead.
 * This interface will be removed in a future version.
 *
 * @category `elicitation/create`
 */
export interface LegacyTitledEnumSchema {
  type: "string";
  title?: string;
  description?: string;
  enum: string[];
  /**
   * (Legacy) Display names for enum values.
   * Non-standard according to JSON schema 2020-12.
   */
  enumNames?: string[];
  default?: string;
}

[mikekistler]

I was going to say yes, but then @stephentoub found that it is still in the spec, so I think we should probably keep it -- sadly.

[stephentoub]

It's also required by the conformance tests:
https://github.com/modelcontextprotocol/conformance/blob/066b2d70800a8bd6bca82e000007c9877b443e5b/src/scenarios/server/elicitation-enums.ts#L20

[jeffhandley]

Keeping this in until it's removed from the spec and conformance tests, meaning first opportunity to delete would be 2.0.0.