Add id to index form

[Flo0807]

Index forms rendered by the built-in fields carry phx-change but no id. Without an id LiveView cannot perform form recovery after a crash or disconnect, and Phoenix.LiveViewTest reports a missing_form_id warning for every render.

This follows #2122 (per-page select form) and #2089 (filter form), which fixed the same class of warning elsewhere.

Changes

Adds an id to the index form in the nine fields that render one: belongs_to, boolean, date, date_time, email, number, select, text, time.

The id reuses the expression already used for the index form input directly below it:

id={"index-form-#{@name}-#{LiveResource.primary_value(@item, @live_resource)}"}

That keeps it unique per field and per item, and correct for resources with a custom or composite primary key.

Also adds an id to the <.form> in the edit_card/1 doc example, so the documented pattern doesn't demonstrate the thing that triggers the warning.

Verification

• mix lint — compile with --warning-as-errors, format, and credo all clean
• MIX_ENV=test mix test — 100 doctests, 174 tests, 0 failures