refactor(api): move file processing polling into SDK-owned helpers (#3712)

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

## Changes being requested

Move the existing sync and async file-processing polling bodies into
SDK-owned `lib/_files.py`. `Files.wait_for_processing` and
`AsyncFiles.wait_for_processing` keep their public signatures, defaults,
docstrings, and return types, and delegate to the helpers. No schema,
compiler, dependency, or generation-metadata changes are needed.

The polling bodies are unchanged apart from renaming the receiver from
`self` to `files`. This preserves the `processed`/`error`/`deleted`
terminal states, wall-clock timing, dynamic `retrieve` and `_sleep`
hooks, return identity, and exact errors. In particular, the strict `>`
timeout check still happens after a repeated retrieval, even if that
retrieval returns a terminal state. This does not introduce a generic
polling framework or change existing timeout behavior.

Review pointers:

- Original public methods:
[sync](https://github.com/openai/openai-python/blob/bedb9a7b8839e193107e88b92f7cc166f08ac83d/src/openai/resources/files.py#L366)
and
[async](https://github.com/openai/openai-python/blob/bedb9a7b8839e193107e88b92f7cc166f08ac83d/src/openai/resources/files.py#L720).
- [Extracted
bodies](https://github.com/openai/openai-python/blob/3dfdf85e4d33a3274270aadccda6278c41ac8ede/src/openai/lib/_files.py#L12).
- [Focused regression
tests](https://github.com/openai/openai-python/blob/3dfdf85e4d33a3274270aadccda6278c41ac8ede/tests/lib/test_file_processing.py#L36):
13 cases covering sync/async terminal results, polling, timeout and
retrieval-error propagation, plus async cancellation.

Following the [review
discussion](https://github.com/openai/openai-python/pull/3712#discussion_r3830698221),
the new handwritten suite is limited to 87 lines rather than an
exhaustive matrix of clock, timeout, and event-order details. Existing
Files API tests are unchanged. The review revision changes only this new
test file; runtime source remains byte-identical to the original PR
head.

## Additional context & links

Validation:

- Commands: `TEST_API_BASE_URL=http://127.0.0.1:4141 .venv/bin/python -m
pytest tests/lib/test_file_processing.py
tests/api_resources/test_files.py -q -n 0` and the same command using
`.venv-pydantic-v1/bin/python`: **130 passed, 3 skipped** in each
Pydantic mode. The skips are the existing aiohttp/respx2-incompatible
`test_method_content`, `test_raw_response_content`, and
`test_streaming_response_content` cases.
- Commands: `.venv/bin/ruff format tests/lib/test_file_processing.py`
and `./scripts/lint` passed on the review revision, including Ruff,
Pyright, mypy, and import checks. The initial full `./scripts/format`
also passed; unrelated reporter-formatting changes were excluded.
- Command: `./scripts/build` passed. Both the wheel and source
distribution contain `openai/lib/_files.py`; importing the helper and
resource classes from the built wheel also passed.
- Exact source/AST comparison against the base verified both polling
bodies, all other resource statements, public signatures, exports,
request construction, and response wrappers.

The verified custom-code report keeps 36 mixed files and changes only
the Files resource customization, shrinking it from **+47/-0 to
+26/-0**. The other 35 customizations are unchanged. Reproduction
command:

```sh
python3 scripts/castiron/custom_code_report.py report \
  --base bedb9a7b8839e193107e88b92f7cc166f08ac83d \
  --head 3dfdf85e4d33a3274270aadccda6278c41ac8ede \
  --fetch --require-head-hash --public \
  --out /tmp/castiron-file-processing
```