`litestar` blocked on free-threaded Python 3.13 (`msgspec` gates `Py_GIL_DISABLED` to 3.14+)

[lesnik512]

litestar unconditionally requires msgspec, which has no free-threaded wheel for 3.13t and fails
to build from source there: msgspec's own _core.c contains
#error "Py_GIL_DISABLED is only supported in Python 3.14+" (v0.21.1). Intentional upstream gate.

Reproduced 2026-07-18: uv pip install --python <3.13t venv> "litestar>=2.9" fails compiling
msgspec._core; the same install against a 3.14t venv succeeds cleanly and the free-threaded smoke
checks pass. The ft CI leg installs per Python version precisely so litestar and
litestar-metrics can run on 3.14t while being excluded only from 3.13t.

Revisit trigger: msgspec extends Py_GIL_DISABLED support to 3.13. Then add
litestar,litestar-metrics to the 3.13t leg's extras in .github/workflows/_checks.yml.