[3/n] Use the standard library for platform headers (#3656)

# Summary

This is part of a series to reduce the SDK’s dependency surface where a
small, purpose-built alternative is sufficient.

- Replace `distro` with Python’s standard OS-release lookup for the
diagnostic platform header.
- Preserve mobile detection, recognize FreeBSD and OpenBSD directly, and
fall back to `Linux` if OS-release information is unavailable.
- Matching `distro`’s exact behavior is explicitly a non-goal. This
header only needs to be close enough for our analytics purposes; we
prefer removing the dependency over reproducing its full
distribution-detection and fallback behavior.
- Remove one runtime dependency: the default graph goes from 15 to 14
distributions, and the all-extras graph from 37 to 36. No remaining
dependency versions change.

## Stack

- https://github.com/openai/openai-python/pull/3653 (merged)
- https://github.com/openai/openai-python/pull/3654 (merged)
- https://github.com/openai/openai-python/pull/3655 (merged)
- https://github.com/openai/openai-python/pull/3656 👈 this PR