Completers as a first class option in Prompts

[mattzcarey]

One thought for a potential follow-up: now that the SDK is moving to schema-library-agnostic types, the completable() API could be simplified to decouple it from Zod entirely.

Instead of embedding completion callbacks inside schema objects (which requires Zod-specific introspection to extract), completers could be passed explicitly in the prompt config:

server.registerPrompt('greeting', {
  argsSchema: type({ name: 'string', language: 'string' }),  // any library
  completers: {
    name: (value) => ['Alice', 'Bob'].filter(n => n.startsWith(value)),
    language: (value) => ['en', 'fr'].filter(l => l.startsWith(value))
  }
}, callback);

The type simplifies too, since completion values are always strings over the wire:

type CompleteCallback = (
  value: string,
  context?: { arguments?: Record<string, string> }
) => string[] | Promise<string[]>;

This PR already does most of the internal plumbing — RegisteredPrompt stores completers?: Map<string, CompleteCallback>. The remaining step would be accepting them in the config directly, which would let us deprecate completable(), extractCompleters, and the Zod-specific helpers (getZodSchemaShape, unwrapZodOptionalSchema, etc.).

Not suggesting this as a blocker for this PR — just flagging as a natural follow-up that would complete the schema-agnostic story.

Originally posted by @felixweinberger in #1473 (comment)