[JacobSzwejbka]

Fix the four Python 3.15 wheel failures from the 1.6.0 RC1 builds, using the existing ExecuTorch fixes in #23876, #23932 and #24238 and pytorch/test-infra#9265.

The wheel build jobs check out test-infra commit b46669025788e540ed4ecc7c56cc52db343e284f, matching ExecuTorch's Windows CUDA pin. This installs final Python 3.15 from conda-forge instead of downloading the broken RC marker package. The reusable release/2.15 workflows and the full Python 3.11–3.15 release matrices remain in place.

The shared pre-build hook exports and persists the PyPI extra index so later smoke installs can resolve packages such as tiktoken. On Linux ARM64 it exposes the existing libatomic runtime through a temporary linker directory and persists that directory for SentencePiece's source build. macOS now runs the same hook, which preinstalls SentencePiece 0.2.2 on Python 3.15 with the libc++ availability workaround scoped to that package's subprocess.

Validation: all four workflows parse; their full version matrices and reusable workflow references are unchanged; all platforms are wired to the hook; bash syntax and git diff checks pass. Running the actual hook in a fresh CPython 3.15.0rc1 environment built a macOS 14 ARM64 SentencePiece wheel from source. Training and Unicode encode/decode checks pass, and the compiler workaround does not leak into the parent environment.

Python 3.15 build and installed-wheel smoke validation is running on an isolated branch. Only the four workflow matrix values differ from this PR, limiting validation to Python 3.15: Linux x64 https://github.com/meta-pytorch/tokenizers/actions/runs/40799730247, Linux ARM64 https://github.com/meta-pytorch/tokenizers/actions/runs/40799735551, macOS ARM64 https://github.com/meta-pytorch/tokenizers/actions/runs/40799740545, Windows x64 https://github.com/meta-pytorch/tokenizers/actions/runs/40799744135.

Authored with Codex assistance.