as("hidden") should allow omitting the value when the field manages its own state #15887

Describe the problem

When using as("hidden") on a remote form field, the value parameter is required — both at the type level (AsArgs) and at runtime (throws in DEV if missing). This forces redundant patterns when the field already manages its own state:

<input {...form.fields.id.as("hidden", form.fields.id.value() ?? "")} />

Other types (text, number, etc.) allow omitting the second argument, in which case they use get_value() internally. Hidden inputs should work the same way.

Describe the proposed solution

Make the value parameter optional for as("hidden"). When omitted, use get_value() (the reactive form state) just like as("text") does:

<!-- Current (required, redundant): -->
<input {...form.fields.id.as("hidden", form.fields.id.value() ?? "")} />

<!-- Proposed (optional, uses field state): -->
<input {...form.fields.id.as("hidden")} />

The explicit value should still be supported for cases where you want to pass an external value (e.g. a prop):

<input {...form.fields.appId.as("hidden", appId)} />

The name prefix (n: for numbers, b: for booleans) can be derived from get_value() at construction time when no explicit value is provided.

Alternatives considered

• Keep the explicit value pattern (current behavior) — works but is verbose and redundant when the field already holds the value via .set()
• Use as("text") and manually add type="hidden":

  <input {...form.fields.id.as("text")} type="hidden" />

  This works since the type attribute overrides the spread, but is a non-obvious workaround that relies on attribute ordering.

Importance

would make my life easier

Additional Information

Related: #14694 (type restrictions for hidden), #15566 / #15577 (declarative values for other types)

SvelteKit 2.60.1, relevant code: src/runtime/form-utils.js lines 711–722