[gen2-migration] Generated DynamoDB table mapping hardcodes the Gen1 env name for branchName with no validation

## Summary
When `generate` produces `amplify/data/resource.ts`, each `migratedAmplifyGen1DynamoDbTableMappings` entry's `branchName` is emitted as a hardcoded string literal of the Gen1 environment name. At runtime `@aws-amplify/backend-data` matches the mapping against the deploy branch (`process.env.AWS_BRANCH ?? "sandbox"`). If the generated literal is not manually edited to match the Gen2 deploy branch, the mapping does not apply and the affected models do not adopt the existing Gen1 tables.

## Details
- `packages/amplify-cli/src/commands/gen2-migration/generate/amplify/data/data.renderer.ts` (~L379) emits `branchName: ''` with only a code comment as guidance.
- The schema branch resolution uses `process.env.AWS_BRANCH ?? "sandbox"`; the two only align if the deploy branch equals the Gen1 env name or the user edits the literal.
- Nothing in the CLI validates or warns on a mismatch.

## Expected
Default the generated `branchName` to the actual deploy branch, and/or emit a validation warning or error at generate/deploy time when the mapping branch cannot match the deploy branch — rather than relying solely on a comment.

## Type
Bug / robustness

Start in packages/amplify-cli/src/commands/gen2-migration/generate/amplify/data/data.renderer.ts around line 379, then trace how generate and deploy resolve the branch name against process.env.AWS_BRANCH. Done means generated mappings no longer silently fail when the deploy branch differs from the Gen1 environment, with the agreed default or a clear validation warning or error.

Written by the indexing model from the issue text.