fix(toolshed): honour the find() -1 sentinel when rewriting generated stub headers

[LeSingh1]

Stacking note: the one-line ci-nightly.yml change (adding toolshed/tests to the tooling test job) is byte-identical to the one in #2539 and #2548, so they merge cleanly in any order.

Problem

_normalize_stub_headers rewrites the OS path separator in a generated stub's first-line comment, so committed .pyi files are identical across platforms. It splits the file on the first newline:

newline = data.find(b"\n")
first_line = data[:newline] if newline != -1 else data
if not first_line.startswith(_HEADER_PREFIX) or b"\\" not in first_line:
    continue
stub.write_bytes(first_line.replace(b"\\", b"/") + data[newline:])

The first slice special-cases the -1 that bytes.find returns when there is no newline. The second does not — and data[-1:] is the file's last byte. For a stub that is a single line with no trailing newline, the rewritten header gets a duplicate of its own final character appended:

in : b'# This file was generated by stubgen-pyx from cuda_core\\cuda\\core\\x.pyx'
out: b'# This file was generated by stubgen-pyx from cuda_core/cuda/core/x.pyxx'
                                                                            ^ duplicated

Two things make this worth fixing rather than shrugging at:

• A .pyi with no trailing newline is a shape this repo tolerates on purpose. .pyi is in the end-of-file-fixer exclude list in .pre-commit-config.yaml, so nothing adds one.
• This wrapper is the stubgen-pyx-cuda-core pre-commit hook — a fixer. The corrupted byte is written straight back into the working tree, and the next git commit takes it.

The -1 handling one line above shows the sentinel was already known about; it just was not applied to both slices.

Fix

Slice the remainder with the same sentinel check:

rest = data[newline:] if newline != -1 else b""

No behavior change for any stub that has a newline, which is every stub in the tree today.

Tests

toolshed/run_stubgen_pyx.py had no tests. Added toolshed/tests/test_run_stubgen_pyx.py, which calls _normalize_stub_headers(tmp_path) directly (no stubgen-pyx binary, no subprocess, no CUDA):

• the separator rewrite with a body and a trailing newline, with a body and no trailing newline, header-only with a newline, and header-only with no newline — asserting the result is byte-for-byte the input with only \ → / changed;
• files that must be left byte-identical: an already-POSIX header (with and without a trailing newline), a hand-written stub containing backslashes but no generated-header prefix, and an empty file.

Verification

Executed in full (pure Python, no GPU):

# with the fix
8 passed

# with toolshed/run_stubgen_pyx.py restored from upstream/main
FAILED test_separator_is_rewritten_without_touching_anything_else[header-only-no-newline]
1 failed, 7 passed

ruff check / ruff format --check clean on both changed Python files. Restore done with cp aside + git show upstream/main:<path> >; index verified clean before committing.

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