Stage 16 — AI Layer: VIN decoder + diagnostic / parts / price helpers

VinDecoder (deterministic, no API):
- ISO 3779/3780 parsing: WMI manufacturer (~60 brands), year (cyclical with
  post-2010 disambiguation via position 7), region, plant, NA checksum
- Strip non-VIN chars, accept dashes/spaces, reject I/O/Q per spec

AiAssistantService:
- Refactored provider HTTP into postClaude/postOpenAI/postGemini so both
  chat history and one-shot calls share the same transport
- singleShot(system, userPrompt, provider?) for fire-and-forget calls
- 4 specialized helpers with tight prompts:
  - suggestDiagnosis(WO) — diagnostician based on complaint + VIN info
  - suggestParts(WO, task) — OEM parts list for an operation
  - suggestPrice(Part) — markup recommendation with justification
  - vinRecommendations(vin, mileage) — scheduled maintenance from decoded VIN
- monthlyUsage() — token spend MTD by provider

Filament:
- VehicleResource: "Decode VIN" + "AI: recomandări" actions
- WorkOrderResource Edit: "AI: sugerează diagnostic" header action
- PartResource: "AI: preț recomandat" action
- Shared views: filament.tenant.ai-reply, filament.tenant.vin-decode
- AiAssistant page shows monthly token usage banner

Tests (13 new):
- 8 VinDecoder unit tests with real VIN samples (Honda 2003, VW 1999, Audi
  2014, Dacia, unknown WMI, lowercase/dashes, forbidden chars)
- 5 AiHelpers feature tests with Http::fake covering all providers + no-key
  fallback + token usage aggregation

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/Filament/Tenant/Resources/WorkOrderResource/Pages/EditWorkOrder.php

@@ -15,6 +15,19 @@ class EditWorkOrder extends EditRecord
     protected function getHeaderActions(): array
     {
         return [
+            Actions\Action::make('ai_diagnose')
+                ->label('AI: sugerează diagnostic')
+                ->icon('heroicon-m-sparkles')
+                ->color('primary')
+                ->visible(fn () => ! empty($this->record->complaint))
+                ->modalHeading('Diagnostic AI bazat pe plângerea clientului')
+                ->modalSubmitAction(false)
+                ->modalCancelActionLabel('Închide')
+                ->modalContent(function () {
+                    [$reply, $meta] = app(\App\Services\Ai\AiAssistantService::class)
+                        ->suggestDiagnosis($this->record);
+                    return view('filament.tenant.ai-reply', ['reply' => $reply, 'meta' => $meta]);
+                }),
             Actions\Action::make('tracking')
                 ->label('Link client (QR)')
                 ->icon('heroicon-m-qr-code')