feat(cli): generate per-table stacks for multiple DDB tables

[9pace]

Summary

• Generate now creates a separate nested stack per DynamoDB table via backend.createStack('storage<resourceName>'), instead of placing all DDB tables in a single shared storage stack. This matches Gen1's per-resource nested stack naming and enables independent refactor/rollback of each table.
• Refactor (forward + rollback) threads the resourceName through StorageDynamoForwardRefactorer and StorageDynamoRollbackRefactorer, using 'storage' + resourceName for both Gen1 and Gen2 nested stack lookups via findNestedStack prefix matching.
• S3 refactorer fix: The S3 forward/rollback refactorers previously used findNestedStack(facade, 'storage') which ambiguously matched DDB per-table stacks (e.g. storageactivity) before the actual S3 stack (storage0EC3F24A). Fixed by using 'storage' + resourceName for Gen1 lookup and a new findS3NestedStack method that distinguishes the CDK-hashed S3 stack from lowercase-prefixed DDB stacks.
• Validation replaces validateSingleResourcePerCategory with validateSingleResourcePerStack, which maps each resource (DDB and S3) to its own stack name and rejects only when multiple resources target the same stack.
• Bug fix: resolveOutputs() no longer crashes when a Gen2 storage stack has no Outputs section (CDK omits Outputs when there are no cross-stack references to a table). CFNTemplate.Outputs is now optional.

Changes

Source (packages/amplify-cli/src/commands/gen2-migration/)

• generate/amplify/backend.generator.ts — new createDynamoDBStack(resourceName) method
• generate/amplify/storage/dynamodb.generator.ts — calls createDynamoDBStack instead of ensureStorageStack
• generate/amplify/storage/dynamodb.renderer.ts — accepts scopeVarName parameter
• refactor/refactor.ts — validateSingleResourcePerStack, threads resourceName into DDB and S3 refactorers
• refactor/storage/storage-forward.ts — accepts resourceName, uses findS3NestedStack for Gen2 lookup
• refactor/storage/storage-rollback.ts — accepts resourceName, uses findS3NestedStack for Gen2 lookup
• refactor/storage/storage-dynamo-forward.ts — per-table stack lookup
• refactor/storage/storage-dynamo-rollback.ts — per-table stack lookup
• refactor/resolvers/cfn-output-resolver.ts — handles missing Outputs section
• cfn-template.ts — Outputs made optional

Tests

• Updated S3 refactorer tests to use realistic stack names (storageavatars, storage0EC3F24A) and pass resourceName
• Updated validation test to expect 'storageavatars' in error message
• New tests for createDynamoDBStack, custom scope variable, multi-DDB generate, no-Outputs templates

Snapshots (amplify-migration-apps/discussions/)

• Fresh snapshots from clean main/gen2-main deployment with auth, API, 2 DDB tables (activity, bookmarks), S3 (avatars), 2 lambdas
• Per-table stacks: storageactivityA346F297, storagebookmarks210DAF62, storage0EC3F24A (S3)

E2E Verification

Tested end-to-end on discussions app (d2l28asetq9wsc, us-east-1):

1. Deploy Gen1 — auth, API (AppSync), 2 DDB tables (activity, bookmarks), S3 (avatars), 2 lambdas
2. Generate — produced createStack('storageactivity') + createStack('storagebookmarks') + S3 storage
3. Deploy Gen2 — separate storageactivityA346F297, storagebookmarks210DAF62, storage0EC3F24A nested stacks
4. Forward refactor — auth (5 resources), activity DDB, bookmarks DDB, and avatars S3 all moved from Gen1 to Gen2
5. Rollback — all resources moved back to Gen1
6. Forward again — full round-trip verified
7. All 389 unit tests pass (48 suites, 67 snapshots)

Test plan

• Unit tests pass (389/389 in 48 suites)
• Snapshot tests pass (67 snapshots)
• E2E: generate produces per-table stacks for multi-DDB app
• E2E: deploy creates separate nested stacks (including S3)
• E2E: forward refactor moves DDB tables and S3 bucket independently
• E2E: rollback restores original Gen1 state
• E2E: forward again succeeds (full round-trip)
• S3 refactorer correctly distinguishes S3 stack from DDB per-table stacks

Closes #14608, #14597

[9pace]

this never happens