Add type-level descriptions to generated Zod schemas #1429

Follow-up from #1416 and #1423.

withDescriptions now emits .describe() for Zod and Zod v4 fields. #1416 also added .describe() to the generated object schema when the GraphQL type itself has a description, for example an input object description.

That type-level part did not land in #1423. It is still useful, especially when callers pass the root schema to tools that read Zod metadata.

Scope I would keep small:

• zod and zodv4 only
• input object schemas, and object/interface schemas when withObjectType is enabled
• both function and const export modes
• escaping coverage for quotes, backslashes, and newlines

The field-level behavior is already done, so this should stay focused on type-level .describe() calls.