Give custom fields a public id

[roncodes]

Every other resource in the platform is addressed by a prefixed public id. A custom field was addressed by its uuid, so any API that hands one out exposes an internal identifier.

FleetOps' inspection API is the case in point: a submission names the field each answer belongs to, and until now had nothing but a uuid to name it with. Fleetbase\Http\Resources\v1\InspectionForm::fieldToArray() carries the comment "A custom field has no public id, so the uuid is the id on both sides" — this removes that compromise.

What changed

• CustomField takes HasPublicId with $publicIdType = 'custom_field', and public_id joins $fillable.
• A migration adds the column and backfills existing rows. It is nullable()->index() rather than unique-and-required, following the groups precedent, so it is safe on an already-populated table.
• HasCustomFields::setCustomField() creates a field with saveQuietly(), which skips the creating hook that mints the id — so that path now mints its own. Without this, a field created on the fly would be the only one without a public id.

Scope

CustomField is platform-wide, so this gives every custom field a public id — orders, vendors, drivers, not only inspections. Nothing reads the new column yet; withCustomFields()'s public projection emits field names and is unchanged.

Checks

• vendor/bin/pest — 1434 passed, 0 failed.
• php-cs-fixer --dry-run — clean.
• New test: generates custom field public ids in tests/Unit/Models/RecordModelsTest.php.
• Two in-memory test schemas gained the column, since the trait's uniqueness probe queries it on every save.

Targeted at release/v1.6.62, cut from main for this change. Left for review — not merging.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (4c763ce) to head (d73e044).

Additional details and impacted files

@@                 Coverage Diff                 @@
##             release/v1.6.62      #254   +/-   ##
===================================================
  Coverage             100.00%   100.00%           
  Complexity              6750      6750           
===================================================
  Files                    398       398           
  Lines                  22502     22505    +3     
===================================================
+ Hits                   22502     22505    +3     

Flag	Coverage Δ
backend	100.00% <100.00%> (ø)

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