Styles: preserve important gradient declarations for statu

[ramonopoly]

Problem: State styles currently append !important directly to CSS values before Style Engine sanitization. For gradient values, this can cause safecss_filter_attr() to reject otherwise valid declarations, so responsive/state gradient CSS may not be emitted.

Solution: Store !important as declaration metadata in the Style Engine, preserve it through rule generation and optimization, and append it only after sanitization.

Backport of ​https://github.com/WordPress/gutenberg/pull/79568

## What

State styles currently append !important directly to CSS values before Style Engine sanitization. For gradient values, this can cause safecss_filter_attr() to reject otherwise valid declarations, so responsive/state gradient CSS may not be emitted.

## How
Store !important as declaration metadata in the Style Engine, preserve it through rule generation and optimization, and append it only after sanitization.

Trac ticket: https://core.trac.wordpress.org/ticket/65561

## Use of AI Tools

AI assistance: Yes
Tool(s): GitHub Copilot, ChatGPT
Model(s): GPT-5.5
Used for: Initial architectural discussion. Unit test scaffolding.

[ramonopoly]

@sachinrajcp123 this ticket already has a patch on ​https://github.com/WordPress/wordpress-develop/pull/12361. Your patch looks like a copy of that, so I think we can safely discard it. Thanks.

I would appreciate it if you could proceed with this PR and commit it in time for the 7.1 Beta1 release on July 15.

I'll rebase this after ​https://github.com/WordPress/wordpress-develop/pull/11384 lands and it should be good to go

[ramonopoly]

In 62720:

Style Engine: Preserve important state declarations

Before, state styles appended !important directly to CSS values before Style Engine sanitization. For gradient values, this caused safecss_filter_attr() to reject otherwise valid declarations, so responsive/state gradient CSS might not have been emitted

This commit stores !important as declaration metadata in the Style Engine, preserves it through rule generation and optimization, and appends it only after sanitization.

Props andrewserong, ramonopoly, wildworks.

Fixes #65561.

Committed in hr62720 / dfff16ff7ef0a06623a35c2aa1743d130b134cc0