gen2-migration retain fails on large AppSync API stacks: CloudFormation 51,200-byte inline templateBody limit (blocks decommission)

## Description

`amplify gen2-migration retain` fails on any project whose generated AppSync API **root** CloudFormation template exceeds **51,200 bytes**, because the command submits the modified template **inline** (`templateBody`) to CloudFormation, which enforces a 51,200-byte limit on inline templates. This blocks the **decommission** step of the official Gen1→Gen2 migration path (`retain` is the documented prerequisite before deleting the Gen1 root stack).

```
🛑 Failed: ValidationError: 1 validation error detected: Value '{...big template...}' at 'templateBody'
failed to satisfy constraint: Member must have length less than or equal to 51200
🛑 Rollback is not supported for the retain step
```

The API root template is large because it inlines the auth-mode `AuthRolePolicy*` `AWS::IAM::ManagedPolicy` resources (one `appsync:GraphQL` statement per field, across all models). A schema with ~20+ `@model` types plus `AWS_IAM` auth easily pushes the root template past 51,200 bytes.

Related: #14790 (the issue that introduced the `retain` command).

## Impact

- `retain` applies `DeletionPolicy: Retain` only to the stacks it processes **before** reaching the oversized API stack, then aborts. In our case it never reached the per-model nested stacks that own the DynamoDB tables.
- Because `retain` "has no rollback" and the update was rejected at validation, no changes are applied (stacks stay `UPDATE_COMPLETE`), but the migration **cannot proceed to a clean Gen1 root-stack deletion** — the documented decommission step is blocked.
- Reproducible and deterministic; affects any sufficiently large API.

## Reproduction

1. Complete a Gen1→Gen2 migration (`assess` → `lock` → `generate` → deploy → `refactor`) for an app with a large AppSync API (many `@model` types + `AWS_IAM` additional auth, so the API root template > 51,200 bytes).
2. Run `amplify gen2-migration retain` from the Gen1 workspace.
3. Observe the `ValidationError` on `templateBody` for the `api` root stack.

## Root cause

`retain` calls CloudFormation `UpdateStack` with an inline `TemplateBody`. CloudFormation limits inline templates to 51,200 bytes; larger templates must be uploaded to S3 and referenced via `TemplateURL` (1 MB limit). The Amplify CLI already uses the S3/`TemplateURL` path for large templates during normal deploys, so the fix is to apply the same approach in the `retain` (and likely `lock`/`refactor`) stack-update code paths.

## Suggested fix

When the modified template exceeds the inline limit, upload it to the deployment bucket and pass `TemplateURL` instead of `TemplateBody`.

## Environment

- `@aws-amplify/cli` 14.5.0 (`gen2-migration retain`)
- region eu-central-1
- Gen1 app migrated to Gen2; ~23 `@model` types, multi-auth (Cognito user pools + `AWS_IAM` + API key).

## Workaround

Skip the stock `retain` and either (a) rely on table-level `DeletionProtectionEnabled` + `delete-stack --retain-resources ` per nested stack, or (b) manually add `DeletionPolicy: Retain` to the stateful resources via S3-based `update-stack`, then delete the root.