fix(drizzle-zod): default TCoerce to undefined in createSchemaFactory

[tsushanth]

Fixes #5968

Root cause

createSchemaFactory has a generic TCoerce with no default:

export function createSchemaFactory<
  TCoerce extends Partial<Record<...>> | true | undefined,
>(options?: CreateSchemaFactoryOptions<TCoerce>)

When the caller omits the coerce option (e.g. createSchemaFactory({ zodInstance: z })), TypeScript cannot infer TCoerce from the argument and falls back to the full constraint bound: Partial<Record<...>> | true | undefined.

This union propagates into GetZodType, where CanCoerce<TCoerce, 'string'> distributes over the union and returns boolean. The conditional type then produces z.ZodCoercedString | z.ZodString for string columns.

BuildRefineField is distributive over its input, so it produces two function overloads with incompatible parameter types:

((schema: z.ZodCoercedString) => z.ZodType) | ((schema: z.ZodString) => z.ZodType) | z.ZodType

TypeScript cannot contextually type a callback against a union of function types with different parameter types, so schema widens to any (ts 7006).

The top-level createInsertSchema / createSelectSchema / createUpdateSchema exports explicitly use CreateInsertSchema<undefined>, which is why they work correctly.

Fix

Add = undefined as the default type parameter for TCoerce in createSchemaFactory. When no coerce option is provided, TCoerce now defaults to undefined instead of widening to the full constraint bound — matching the behaviour of the top-level exports.