`POST /api/requests/batch` checks that `requests` is a non-empty list
and then hands each element to the shared preparation helper, which
calls `.get()` on it. A bare string, number or null in the list raises
`AttributeError` and the caller gets a 500, while `POST /api/requests`
answers 400 with a message for the same mistake.
This validates the element type beside the existing list check. One bad
item rejects the whole batch rather than being reported per item, which
matches the endpoint's current contract: every other failure path
already aborts the batch with a single error body, as
`test_batch_create_requests_is_atomic` asserts. Responses for valid
payloads are unchanged.
## Verification
- `tests/core/test_request_routes_api.py`: a case per bad shape (int,
str, null, list), plus a mixed valid and invalid batch that asserts
nothing was created. All fail on current main with a 500 and pass here.
- Full suite (3168), ruff, ruff format, basedpyright, vulture green.
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