chore(elicitation): improve enum schema builder, small changes of elicitation builder

[pbezglasny]

Motivation and Context

Improve enum elicitation builder: switch to typesage implementation with better readability

Also:

• set legacy return type for required_enum/optional_enum to match previous version return type
• fix deprecation version to correct next version

How Has This Been Tested?

Unit/integration tests

Breaking Changes

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

Additional context

[alexhancock]

Can you provide more context on the motivation behind the changes?

[pbezglasny]

@alexhancock previously in #539 added builder for eliciation enum. I rewrite it in more type safe manner to improve user experience. Also, small fixes before 0.13 release to backward capability with 0.12.

[alexhancock]

@pbezglasny Got it. I had already cut for #620

Are the backwards compat improvements worth re-cutting?

[pbezglasny]

@alexhancock
Well only difference, that

rust-sdk/crates/rmcp/src/model/elicitation_schema.rs

Line 1404 in e49aef6

pub fn required_enum(self, name: impl Into<String>, values: Vec<String>) -> Self {

now return legacy format for fully backward capability (format described here),

// Continue to support the current EnumSchema as Legacy

/**
  * Legacy: Use TitledSingleSelectEnumSchema instead. 
  * This interface will be removed in a future version.
  */
export interface LegacyEnumSchema {
  type: "string";
  title?: string;
  description?: string;
  enum: string[];
  enumNames?: string[]; // Titles for enum values (non-standard, legacy)
}

in the cut it return new format for untitled singlar enum

// Single select enum without titles 
export type UntitledSingleSelectEnumSchema = {
      type: "string";
      title?: string;
      description?: string;
      enum: string[]; // Plain enum without titles
    };

It's basically the same schema, so it's supposed to work as is.