Fix src/msgspec/structs.pyi typings - #1065

Widening the runtime to accept `None` was the alternative. I rejected
it: the docstring specifies `offset : int, optional` and reserves `-1`
as the sentinel for writing at the end of the buffer, which leaves
`None` with no meaning to carry. Narrowing the stub makes the declared
type match both the C code and the documented contract.

The `int | None` traces back to the original `Optional[int]` annotation
and survived the 3.10+ syntax migration in msgspec#1041. Nothing in the tree
passes `None`.

Residual limitation: this edits the same two stub lines as open PR
msgspec#1160, which reshapes `buffer` and the return type while keeping
`offset: int | None`. Whichever lands second needs a one-line rebase.
The two changes are otherwise independent, since msgspec#1160 keeps reading
`offset` via `PyLong_AsSsize_t`.

Measurement, CPython 3.14:

- `just test-typing` clean under mypy 2.3.0, pyright 1.1.411, pyrefly
1.2.0.
- Reverting the stub hunk alone makes all three checkers fail on both
new call sites in `basic_typing_examples.py`, so the type-level test is
not vacuous.
- `pytest tests/unit`: 6119 passed, 404 skipped, identical to the
pre-change baseline. The two runtime assertions are pins on current
behavior rather than new coverage.
- `ruff format` collapses the signature onto one line once `| None` is
gone; that reformat is included.

No changelog entry, following merged stub PRs msgspec#1055, msgspec#1057, msgspec#1062,
msgspec#1065, msgspec#1074 and msgspec#1093, which leave it to the aggregated "type stub
improvements" line at release.

Co-authored-by: Dylan Pulver <dylanpulver@users.noreply.github.com>