fix(compiler): avoid leaking ::ng-deep detection state between rules - #71119

PR Checklist

• The commit message follows Angular's guidelines.
• Tests for the changes have been added.
• Docs have been added / updated (not applicable: no API change).

PR Type

• Bugfix

What is the current behavior?

#69885 skips encapsulating nested selectors when the parent selector contains ::ng-deep. The check uses _shadowDeepSelectors.test(part), but _shadowDeepSelectors is a module-level regex with the g flag, so a match leaves lastIndex set for the next call.

When the matched rule has an empty body, nothing resets it. The next ::ng-deep rule is then tested from the stale offset, is not detected, and its nested selectors get scoped. This also crosses stylesheets: with @angular/compiler@22.2.1,

encapsulateStyle('::ng-deep .p { .q { color: blue; } }', 'b');
// .p { .q { color: blue; } }

encapsulateStyle('.foo ::ng-deep .bar {}', 'a');
encapsulateStyle('::ng-deep .p { .q { color: blue; } }', 'b');
// .p { .q[_ngcontent-b] { color: blue; } }

Issue Number: N/A

What is the new behavior?

The check uses a non-global copy of the same pattern, so the result no longer depends on earlier rules. _shadowDeepSelectors keeps the g flag for its replace and split uses.

Does this PR introduce a breaking change?

• No

Other information

• pnpm bazel test //packages/compiler/test:test — the new spec fails without the fix (.qux[contenta]) and passes with it; the rest of the suite passes.
• Prettier and commit message checks passed.