fix: keep a persisted field's code stable when its name is renamed [3.x]

[ManukMinasyan]

Problem

The name input on FieldForm derives code from name whenever the current code still matches the slug of the previous name:

if (($get('code') ?? '') !== Str::of($old)->slug('_')->toString()) {
    return;
}

$set('code', Str::of($state)->slug('_')->toString());

That heuristic is correct while a field is being created — it stops overwriting a code the user typed by hand. On a persisted field it is wrong: the code is an identity, not a label. Stored values, report columns and visibility conditions all key on it, and a field cloned onto a new form version deliberately shares its code with the original.

So renaming a field silently rewrote its code and severed it from its own data: the existing values kept the old code, and reporting then offered two unrelated columns where the user expected one renamed column.

Fix

Only derive the code while the field is being created, detected by the absence of a bound record.

This is correctly scoped across all four FieldForm::schema() consumers:

Consumer	Binds a record?	Behaviour
ManageCustomFieldSection::createField	no	derivation still runs
ManageFieldsTable create action	no	derivation still runs
ManageCustomField::editAction	yes	derivation suppressed
ManageFieldsTable::editField	yes	derivation suppressed

duplicateAction does not use this form — it is confirmation-only and already generates its own unique code via generateUniqueCode(), so it is unaffected.

Verification

New regression test in CustomFieldsFieldManagementTest, verified red against the previous implementation (it failed on the code assertion) and green with the fix.

• Full package suite: 786 passed, 3 todos, 0 failures
• PHPStan: no errors
• Rector + Pint: clean

Downstream

This is the blocker for Padmission/journey#3427, where renaming a field on a new Flex Fund form version broke code continuity with v1 and hid the field from Custom Reports. Journey will need a version bump once this is released.

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