Relax msgspec upper bound for Python 3.15

[Filimoa]

The msgspec<=0.21.1 upper bound prevents installing msgspec 0.22.0, which provides Python 3.15 wheels. Remove the upper bound while retaining msgspec>=0.18 so applications can resolve a compatible release.

Update the lockfile to select msgspec 0.22.0 on Python 3.10+, retaining 0.18.6 for Python 3.8 and 0.20.0 for Python 3.9. The package version is unchanged.

Validation on macOS ARM64 with Python 3.15.0rc2 and msgspec 0.22.0:

• All 15 existing tests in tests/test_core.py passed using a placeholder API key (no live API calls).
• Serializer smoke checks passed for request/response dictionaries, enum conversion, and independent mutable defaults.
• uv lock --check --offline and git diff --check passed.

Live Capsolver API tests were not run.