fix: update form overload to set invalid type to schema input

[sillvva]

Discussed in #14745. The Invalid<Input> generic should be automatically based on the schema input type instead of the output type.

I and others have run into the issue where we're coercing date strings to actual Date objects, and that causes a type error when trying to do something like invalid.date("Invalid date"). In this case, invalid.date is of type never, because it only accepts string | number | boolean | File types. If it were based on the schema input type, that wouldn't be an issue.

const schema = z.object({
  date: z.coerce.date<string>(),
});

// Doesn't work. `invalid` is based on schema output type.
// It only accepts types: string | number | boolean | File
export const testForm = form(schema, (input, invalid) => {
  // input: Invalid<{ date: string }>
  // output: Invalid<{ date: Date }>
  invalid(invalid.date('Not valid'));
  //              ^^^^ Type 'never' has no call signatures.
});

---

Please don't delete this checklist! Before submitting the PR, please make sure you do the following:

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with pnpm test and lint the project with pnpm lint and pnpm check

Changesets

• If your PR makes a change that should be noted in one or more packages' changelogs, generate a changeset by running pnpm changeset and following the prompts. Changesets that add features should be minor and those that fix bugs should be patch. Please prefix changeset messages with feat:, fix:, or chore:.

Edits

• Please ensure that 'Allow edits from maintainers' is checked. PRs without this option may be closed.

[changeset-bot]

🦋 Changeset detected

Latest commit: 18a5fc0

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@sveltejs/kit	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-14748-svelte.vercel.app/

[dummdidumm]

thank you!