fix: custom realtime models broken under zod 4.4+

[AdirAmsalem]

Summary

A fresh install of @decartai/sdk started failing realtime connect for users who pass a custom model definition without an inputSchema. The error surfaces as:

[ { code: "invalid_type", expected: "nonoptional",
    path: ["model","inputSchema"],
    message: "Invalid input: expected nonoptional, received undefined" } ]

This regression is triggered by zod 4.4, which stopped treating z.any() as implicitly optional inside object schemas. Built-in models (models.realtime("lucy-2.1"), etc.) were unaffected because they always set inputSchema. Users following our own examples/sdk-core/realtime/custom-model.ts pattern were broken.

The fix aligns the runtime schema with the existing TypeScript type (CustomModelDefinition.inputSchema?) and works under zod 4.0–4.4+.

Usage (unchanged, now actually works on zod 4.4+)

const customModel: CustomModelDefinition = {
  name: "my-preview-model",
  urlPath: "/v1/stream",
  fps: 20,
  width: 1280,
  height: 720,
};

await client.realtime.connect(stream, { model: customModel, onRemoteStream });

Test plan

• Reproduce the user error against zod@4.4.1 with the old schema (matches reported error verbatim)
• Verify fixed schema accepts custom models with and without inputSchema under zod@4.4.1
• Verify built-in models still validate
• Verify the schema still rejects truly-invalid model definitions (missing fps, etc.)
• pnpm --filter @decartai/sdk typecheck
• pnpm --filter @decartai/sdk test (175/175 pass)

---

Note

Low Risk
Low risk, single-field validation change that relaxes runtime model definition parsing to match existing TS types; main impact is allowing previously-rejected custom realtime model configs under newer Zod versions.

Overview
Fixes a regression in @decartai/sdk model validation by making modelDefinitionSchema.inputSchema optional, aligning runtime Zod validation with CustomModelDefinition.inputSchema?.

This restores compatibility for custom model definitions (notably realtime connect) that omit inputSchema, which started failing validation under Zod 4.4+.

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commit: 27d52eb