CSS: Fix string token value on special string escapes

[sirreal]

Summary

• Fix \-newline and \-EOF handling in CSS string token value decoding (fixes CSS Processor string token values mishandle backslash-newline #222, CSS Processor string token values mishandle backslash-EOF #223)
• Rename decode_string_or_url → decode_escapes with a new $string_escapes parameter for CSS string token rules (§4.3.5): \-newline discards both characters (line continuation), \-EOF discards the backslash
• Add test coverage for these cases and fix incorrect expected values in the JSON test corpus
• Add css test group to allow vendor/bin/phpunit --group css

Note that the css test file diverged from the originals in these cases. For example, see string/0005 where the expected token value is foo like in this PR.

Also see #226 where bad-string is updated to use null token value. These changes will conflict and require an update assuming both are expected to land.

Test plan

CI passes. Visually confirm that updated and new tests are correct.

[sirreal]

This diverges from #226 and will need

-			self::TOKEN_STRING === $this->token_type || self::TOKEN_BAD_STRING === $this->token_type
+			self::TOKEN_STRING === $this->token_type

[Copilot]

Pull request overview

This PR updates the CSS tokenizer’s escape decoding to follow the CSS Syntax spec’s special handling for string tokens, specifically fixing how \ + newline and \ + EOF contribute to decoded string values, and aligns the test corpus accordingly.

Changes:

• Implement special string-token escape handling (\-newline → discard both, \-EOF → discard backslash) via decode_escapes(..., $string_escapes=true).
• Update the JSON test corpus expected normalized/value fields for affected cases.
• Add/extend PHPUnit coverage for string/URL/ident backslash-newline and backslash-EOF behaviors, and introduce a css PHPUnit group.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
components/DataLiberation/CSS/class-cssprocessor.php	Renames and extends escape decoding to support string-token-specific escape rules; updates call sites.
components/DataLiberation/Tests/CSSProcessorTest.php	Adds targeted regression tests for backslash-newline and backslash-EOF across string/URL/ident contexts; adds @group css.
components/DataLiberation/Tests/css-test-cases.json	Fixes expected normalized/value outputs for string tokens containing backslash-newline and backslash-EOF.
components/DataLiberation/Tests/CSSUrlProcessorTest.php	Adds @group css to allow running CSS-related tests via a PHPUnit group.

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[Copilot]

The decode_escapes() docblock still says it “Decodes a string or URL value”, but the method is now used for identifiers, functions, hashes, delims, etc. This makes the documentation misleading; update the summary/param docs to describe it as generic token-slice escape decoding + CSS normalization (with optional string-token special rules when $string_escapes is true).