Dashboard migration: Implement v2 to v0 conversions (#114812)

* Update docs

* Remove 406 response since now it is converted

* fix linter

---------

Co-authored-by: Stephanie Hingtgen <stephanie.hingtgen@grafana.com>

apps/dashboard/pkg/migration/README.md

@@ -8,12 +8,13 @@ This document describes the Grafana dashboard migration system, focusing on conv
   - [Conversion Flow](#conversion-flow-v0--v1--v2)
   - [v0 to v1 Conversion](#v0-to-v1-conversion)
   - [v1 to v2 Conversion](#v1-to-v2-conversion)
+  - [v2 to v0/v1 Conversion](#v2-to-v0v1-conversion)
 - [Conversion Matrix](#conversion-matrix)
 - [API Versions](#api-versions)
 - [Schema Versions](#schema-versions)
 - [Testing](#testing)
   - [Backend conversion tests](#backend-conversion-tests)
-  - [Frontend migration comparison tests](#frontend-migration-comparison-tests)
+  - [Backend and frontend conversion parity tests](#backend-and-frontend-conversion-parity-tests)
 - [Monitoring Migrations](#monitoring-migrations)
   - [Metrics](#metrics)
     - [Dashboard conversion success metric](#1-dashboard-conversion-success-metric)
@@ -54,6 +55,12 @@ v0alpha1 (Legacy JSON) → v1beta1 (Migrated JSON) → v2alpha1/v2beta1 (Structu
 - Handles modern dashboard features and Kubernetes-native storage
 - See [V2 to V1 Layout Conversion](./conversion/v2_to_v1_layout_conversion.md) for details on how V2 layouts are converted back to V1 panel arrays
 
+#### v2 to v0/v1 Conversion:
+- Converts structured v2 dashboards back to JSON format (v0alpha1 or v1beta1)
+- Chains through intermediate versions: v2 → v1beta1 → v0alpha1
+- v0alpha1 and v1beta1 share the same spec structure (only API version differs)
+- Enables backward compatibility when storing v2 dashboards in legacy format
+
 ## Conversion Matrix
 
 The system supports conversions between all dashboard API versions:
@@ -113,25 +120,51 @@ go test ./apps/dashboard/pkg/migration/conversion/... -v
 go test ./apps/dashboard/pkg/migration/... -run TestSchemaMigrationMetrics
 ```
 
-### Frontend migration comparison tests
+### Backend and frontend conversion parity tests
 
-The frontend migration comparison tests validate that backend and frontend conversion logic produce consistent results:
+These tests ensure that backend (Go) and frontend (TypeScript) conversions produce identical outputs. This is critical because:
 
-- **Test methodology**: Compares backend vs frontend conversion outputs through DashboardModel integration
-- **Dataset coverage**: Tests run against curated test files covering various conversion scenarios
-- **Test location**: `public/app/features/dashboard/state/DashboardMigratorToBackend.test.ts`
-- **Test data**: Located in `apps/dashboard/pkg/migration/testdata/input/` and `testdata/output/`
+- **Dual implementation**: Both backend and frontend implement dashboard version conversions
+- **Consistency requirement**: Users should see the same dashboard regardless of which path is used
+- **API flexibility**: The API may return dashboards in different versions depending on context
+
+**Why normalize through Scene?**
+
+Both backend and frontend outputs are passed through the same Scene load/save cycle before comparison. This normalization:
+- Eliminates differences from default values added by Scene
+- Handles field ordering variations
+- Simulates the real-world flow: dashboard loaded → edited → saved
+
+**Test locations:**
+
+| Test File | Purpose |
+|-----------|---------|
+| `public/app/features/dashboard-scene/serialization/transformSaveModelV1ToV2.test.ts` | v1beta1 → v2beta1 conversion parity |
+| `public/app/features/dashboard-scene/serialization/transformSaveModelV2ToV1.test.ts` | v2beta1 → v1beta1/v0alpha1 conversion parity |
+| `public/app/features/dashboard/state/DashboardMigratorToBackend.test.ts` | Schema version migration parity |
 
 **Test execution:**
 ```bash
-# Frontend migration comparison tests
+# V1 to V2 conversion parity tests
+yarn test transformSaveModelV1ToV2.test.ts
+
+# V2 to V1 conversion parity tests
+yarn test transformSaveModelV2ToV1.test.ts
+
+# Schema migration parity tests  
 yarn test DashboardMigratorToBackend.test.ts
 ```
 
-**Test approach:**
-- **Frontend path**: `jsonInput → DashboardModel → DashboardMigrator → getSaveModelClone()`
-- **Backend path**: `jsonInput → Backend Conversion → backendOutput → DashboardModel → getSaveModelClone()`
-- **Comparison**: Direct comparison of final converted states from both paths
+**Test approach (v1 → v2):**
+- **Backend path**: `v1beta1 → Go conversion → v2beta1 → Scene → normalized output`
+- **Frontend path**: `v1beta1 → Scene → v2beta1 → Scene → normalized output`
+- **Test data**: Uses files from `apps/dashboard/pkg/migration/conversion/testdata/` and migrated dashboards
+
+**Test approach (v2 → v1/v0):**
+- **Backend path**: `v2beta1 → Go conversion → v1beta1/v0alpha1 → Scene → normalized output`
+- **Frontend path**: `v2beta1 → Scene → v1beta1 → Scene → normalized output`
+- **Target versions**: Tests both v0alpha1 and v1beta1 (they share the same spec structure)
+- **Test data**: Uses files from `apps/dashboard/pkg/migration/conversion/testdata/`
 
 For schema version migration testing details, see the [SchemaVersion Migration Guide](./schemaversion/README.md).