refactor(api): isolate vector-store polling helpers (#3713)

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

## Changes being requested

Move the existing sync/async vector-store file and file-batch polling
bodies into SDK-owned `lib/_vector_stores.py`, sharing their identical
`openai-poll-after-ms` lookup, integer conversion, and one-second
fallback. The public `poll` methods remain thin delegates with the same
signatures, defaults, docstrings, and return types.

Request headers, response parsing, interval handling, terminal
conditions, return identity, and error/cancellation propagation are
preserved. Upload orchestration, create-and-poll methods, other
requests, and response wrappers are untouched. This adds no generic
polling framework, timeout behavior, schema/compiler changes, or
generation-metadata changes.

Review pointers:

- Original public polling bodies:
[Files](https://github.com/openai/openai-python/blob/bedb9a7b8839e193107e88b92f7cc166f08ac83d/src/openai/resources/vector_stores/files.py#L360)
and
[FileBatches](https://github.com/openai/openai-python/blob/bedb9a7b8839e193107e88b92f7cc166f08ac83d/src/openai/resources/vector_stores/file_batches.py#L331),
with the corresponding async methods in the same files.
- [SDK-owned
helper](https://github.com/openai/openai-python/blob/25e95b31c1402b3a927cedb9fff5d3d7a785bbeb/src/openai/lib/_vector_stores.py#L16):
the four bodies only rename their receiver and replace the repeated
interval block with the shared lookup. Source/AST comparison against the
base verifies those edits and the unchanged remainder of both resources.
- [Focused
tests](https://github.com/openai/openai-python/blob/25e95b31c1402b3a927cedb9fff5d3d7a785bbeb/tests/lib/test_vector_store_polling.py#L47):
106 lines / 22 cases covering the four public entry points, terminal
results, default/server/explicit intervals and headers, retrieval
errors, and async cancellation. Existing API and helper-signature tests
are unchanged.

## Additional context & links

Validation:

- Commands: `.venv/bin/python -m pytest
tests/lib/test_vector_store_polling.py -q -n 0` and the same command
with `.venv-pydantic-v1/bin/python` passed all **22 cases against the
original implementation and again after extraction**.
- Commands: `TEST_API_BASE_URL=http://127.0.0.1:4142 .venv/bin/python -m
pytest tests/api_resources/vector_stores
tests/lib/test_vector_store_files.py
tests/lib/test_vector_store_file_batches.py
tests/lib/test_vector_store_polling.py -q -n 0` and the same command
with `.venv-pydantic-v1/bin/python` passed **248 tests in each Pydantic
mode**.
- Commands: `./scripts/format` and `./scripts/lint` passed, including
Ruff, Pyright, mypy, and import checks. Unrelated reporter-formatting
edits were excluded.
- Command: `./scripts/build` passed. Both distributions include the
helper, and a fresh import from the built wheel loads the helper and all
four resource classes.

The verified custom-code report keeps **36 mixed files**, with only the
two vector-store resource customizations changed:

- `files.py`: **+222/-4 to +179/-1**.
- `file_batches.py`: **+275/-4 to +242/-3**.

All other 34 customizations, generation metadata, API reference,
dependencies, and workflows are unchanged. Reproduction command:

```sh
python3 scripts/castiron/custom_code_report.py report \
  --base bedb9a7b8839e193107e88b92f7cc166f08ac83d \
  --head 25e95b31c1402b3a927cedb9fff5d3d7a785bbeb \
  --fetch --require-head-hash --public \
  --out /tmp/castiron-vector-polling
```