[ast] 1b: Extract rendered-text builders into utils/codegen.ts

Part of #3572 (Epic 1: separate concerns in the `@kubb/ast` utils layer). ## Goal Move the helpers that assemble rendered source text out of `utils/index.ts` into `src/utils/codegen.ts`. ## Functions to move (from `src/utils/index.ts`) - `buildJSDoc` - `buildObject` - `buildList` - `objectKey` - `indentLines` (currently private; keep it **internal** to the new module, do not export it) Note: `buildObject`/`buildList` use `indentLines` and `INDENT` from `../constants.ts`; keep those imports. ## Files - New: `packages/ast/src/utils/codegen.ts` (+ `codegen.test.ts`). - Edit: `packages/ast/src/utils/index.ts` — re-export `buildJSDoc`, `buildObject`, `buildList`, `objectKey`. ## Acceptance - `@kubb/ast/utils` still exports `buildJSDoc`, `buildObject`, `buildList`, `objectKey`; `indentLines` stays internal. - `@kubb/ast` green; plugin snapshots byte-identical. ## Dependencies None. Parallel-safe with other Epic 1 sub-issues.