Add type-level Zod descriptions

[Code-Hex]

What changed

withDescriptions: true already emitted .describe(...) for Zod fields. This adds the same treatment to the generated schema itself when the GraphQL type has a description.

Covered cases:

• zod and zodv4
• input object schemas
• object and interface schemas when withObjectType is enabled
• function and const export modes
• @oneOf input objects
• strict object schemas, so the output becomes .strict().describe(...)

The description text uses JSON.stringify, matching the field-level path and keeping quotes, backslashes, and newlines valid in the generated TypeScript.

Fixes #1429.

Checks

• pnpm vitest run tests/zod.spec.ts tests/zodv4.spec.ts
• pnpm test
• pnpm type-check
• pnpm lint