fix: fix RecursiveFormFields type for recursive or unknown schemas

[sillvva]

fixes #14687

This ensures form.fields.allIssues() always exists, even if T in RemoteFormFields<T> is unknown or otherwise indefinitely recursive.

Example:

type Test1 = RemoteFormFields<unknown>["allIssues"]; // ✅ after, ❌ before 
type Test2 = RemoteFormFields<unknown>["foo"]["as"]; // ✅ after, ✅ before 

Nested unknown properties will still be treated as leaves, aka RemoteFormField. For example, if form.fields.nested is of type unknown, it will be considered a leaf and have the as method.

Example:

type Nested = { foo: { bar: string; baz: unknown; } };
type Test3 = RemoteFormFields<Nested>["allIssues"]; // ✅ after, ✅ before
type Test4 = RemoteFormFields<Nested>["foo"]["allIssues"]; // ✅ after, ✅ before
type Test5 = RemoteFormFields<Nested>["foo"]["bar"]["as"]; // ✅ after, ✅ before
type Test6 = RemoteFormFields<Nested>["foo"]["baz"]["as"]; // ✅ after, ✅ before

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