[Zod]: Default createSchemaFactory's coercion generic so refine callbacks keep their types

[ShreeBohara]

Fixes #5968.

createSchemaFactory declares TCoerce extends CoerceOptions with no default, so any call that
omits the coerce key leaves TCoerce at the constraint itself rather than resolving to a concrete
option set. That constraint then distributes through the refine field types into a union of callbacks
with different parameter types, and TypeScript can no longer contextually type the callback:

const { createSelectSchema } = createSchemaFactory({ zodInstance: z });

createSelectSchema(table, {
  c2: (schema) => schema.lte(1000),
  //   ^ TS7006: Parameter 'schema' implicitly has an 'any' type.
});

It reproduces with createSchemaFactory(), createSchemaFactory({}), and
createSchemaFactory({ zodInstance: z }) — anything without coerce. Passing coerce: true or an
object works today, which is why the existing factory tests don't catch it: they all specify
coerce.

Defaulting the parameter to undefined matches what the top-level createSelectSchema,
createInsertSchema and createUpdateSchema exports already do — they are typed
CreateSelectSchema<undefined> and friends — and it matches the runtime, which treats a missing
coerce as {}. Explicit coerce: true and coerce: { date: true } still infer exactly as before.

The test is added to integration-tests/tests/validators/zod/pg.test.ts next to the existing
coercion cases, and mirrors the shape of the refine table - select test already there. Against the
current rc5 source it reports three errors without the change (the TS7006 above, plus the
downstream argument and Expect<Equal<...>> failures) and none with it.

drizzle-orm package tests pass: 1223 passed, 7 skipped. dprint and oxlint are clean on both
touched files. The type-tests suite reports 52 errors both with and without this change — they all
come from effect/postgres typings in my local install and are byte-identical on a clean rc5
checkout, so they're unrelated.

Two earlier PRs made the same one-line change, #5973 and #6031, but both targeted main and edited
drizzle-zod/src/schema.ts, which no longer exists on this branch; both were closed by their authors
without review. This one is against rc5 and adds the regression test.

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