Allow negated prefers-reduced-motion media queries in AMP validator

[Durvesh-dev]

Summary

Fixes AMP validator incorrectly flagging valid CSS media queries using
not (prefers-reduced-motion: reduce) as malformed.

Details

• Adds not as a valid media query modifier in the validator spec
• Adds a validator CSS test to prevent regressions

Motivation

This syntax is valid per CSS Media Queries Level 5 and is already used in
AMP documentation (e.g. amp-animation).

Fixes #40384

[CLAassistant]

All committers have signed the CLA.

[Durvesh-dev]

Hi @erwinmombay 👋

This PR updates the AMP validator to allow negated
prefers-reduced-motion media queries, which are already documented
and used in AMP components.

All CI checks are passing.
Could you please take a look when you have time?

Thanks!

[milindmore22]

LGTM

[erwinmombay]

thanks @Durvesh-dev , reviewing.

[banaag]

Could you add a validator test for this? Something like: https://github.com/ampproject/amphtml/pull/39558/files#diff-cf571d8f18ca528dbc7956b0a627f9f486ef84b415beaf3ba4a4d6355b4cb4bf

[erwinmombay]

@Durvesh-dev you can probably add it here https://github.com/ampproject/amphtml/blob/004f2d2ecf03f0e899b67c5b382c4760f3b6b8a7/validator/testdata/feature_tests/everything.html