chore(api): move webhook tests out of generated code (#3700)

- [x] I understand that this repository is auto-generated and my pull
request may not be merged

## Changes being requested

Move the handwritten webhook regression suite from the generated
resource-test
file to `tests/lib/test_webhooks.py`. The new file is byte-for-byte
identical to
the old file after removing its generated-file header. The generated
path
returns to the verified Castiron output.

All 71 collected cases remain. The existing synthetic signature
fixtures, time
mocks, secret/header validation, timestamp tolerance, byte payloads,
multiple
signatures, exceptions, and sync/async client modes are unchanged. No
webhook
verification code, public API, or generation metadata changes are
included.

## Additional context & links

Coverage now lives in these exact classes:

-
[`tests/lib/test_webhooks.py::TestWebhooks`](https://github.com/openai/openai-python/blob/c69d47ae202259ed693a8ae230e97ab988371bfd/tests/lib/test_webhooks.py#L32):
  16 methods, 31 collected cases.
-
[`tests/lib/test_webhooks.py::TestAsyncWebhooks`](https://github.com/openai/openai-python/blob/c69d47ae202259ed693a8ae230e97ab988371bfd/tests/lib/test_webhooks.py#L170):
  14 methods, 40 collected cases, including aiohttp.

<details>
<summary>Exact preserved test methods</summary>

Both classes retain:

- `test_unwrap_with_secret`
- `test_unwrap_without_secret`
- `test_unwrap_with_client_secret`
- `test_verify_signature_valid`
- `test_verify_signature_invalid_secret_format`
- `test_verify_signature_invalid`
- `test_verify_signature_missing_webhook_signature_header`
- `test_verify_signature_missing_webhook_timestamp_header`
- `test_verify_signature_missing_webhook_id_header`
- `test_verify_signature_payload_bytes`
- `test_verify_signature_timestamp_too_old`
- `test_verify_signature_timestamp_too_new`
- `test_verify_signature_multiple_signatures_one_valid`
- `test_verify_signature_multiple_signatures_all_invalid`

`TestWebhooks` also retains `test_verify_signature_custom_tolerance` and
`test_verify_signature_recent_timestamp_succeeds`.

</details>

Validation:

- Pytest collection matches all 71 original node IDs after replacing
only the
  module path, in both Pydantic modes.
- Command: `python -m pytest -q -n 4
tests/api_resources/test_webhooks.py tests/lib/test_webhooks.py`
  passed 71 tests under Pydantic v2 and 71 under Pydantic v1.
- `./scripts/format` and `./scripts/lint` passed, including Ruff,
Pyright, mypy,
  and import checks. Unrelated reporter formatting is excluded.
- The public custom-code report verifies 41 -> 40 mixed files, one
removed
customization, and no other changed customizations.
`.castiron.stats.yml`
  is unchanged.

For a byte-level check, both commands return
`09e457aee921b3c53c64485fc235724ea4ed5402`:

```sh
$ git show 8edd9ae411f9d0a5385447a4697c9f7042868213:tests/api_resources/test_webhooks.py | tail -n +3 | git hash-object --stdin
$ git rev-parse c69d47ae202259ed693a8ae230e97ab988371bfd:tests/lib/test_webhooks.py
```