chore(api): remove redundant generated formatting (#3696)

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

## Changes being requested

Remove redundant import wrapping in five generated resources and one
extra
blank line in `MessageContent`. All six files become byte-identical to
the
verified Castiron output, with unchanged Python ASTs.

The verified local custom-code report drops from 52 to 46 mixed files.
No
runtime behavior, public signatures, imports, exports, dependencies, API
schema, or compiler behavior changes.

## Additional context & links

The cleanup survives a fresh Castiron generation using the existing API
and
configuration inputs. The old and new pure-generated content hashes
match.
No Stainless fixture or generated-file ownership rule changes.

Validation:

- Pinned Ruff formatting and lint checks passed for all six files.
- `./scripts/format` preserved the cleanup; unrelated formatter-only
changes
  to the existing report scripts were not included.
- `./scripts/lint` passed, including Pyright, mypy, and import checks.
- The affected Threads Messages, Completions, Fine-tuning
Jobs/Checkpoints,
  and Models resource suites passed: 380 tests under Pydantic v2 and 380
  under Pydantic v1, using the local mock server.
- The custom-code reporter verified the generated baseline and removed
  exactly these six customizations.

No compiler companion is needed. This is a behavior-preserving SDK
cleanup.

Co-authored-by: apcha-oai <228803254+apcha-oai@users.noreply.github.com>