Improve CSS parse error recovery and reporting (#90025)
## What?
This PR improves CSS parsing error handling in Turbopack by making parse errors recoverable instead of fatal.
1. **Extracting CSS parsing logic** into a new `parse_css_stylesheet` function that handles both parsing and CSS module validation in one place
2. **Fixing error recovery** when CSS minification fails — instead of immediately returning an unparsable result, the code now re-parses the original CSS to recover valid styles
3. **Improving error reporting** by renaming variables for clarity (`source` → `issue_source`) and ensuring errors are properly emitted before attempting recovery
4. **Adding comprehensive tests** for CSS parse error scenarios with both snapshot tests and e2e tests
The snapshot test fixture exercises several CSS features to confirm they survive error recovery:
- Basic selectors (`.before`, `.after`) — valid rules are preserved when surrounded by invalid ones
- `::highlight(highlight-test)` — CSS Custom Highlight API pseudo-element
- `scroll-marker-group`, `::scroll-marker`, `::scroll-marker:target-current` — CSS scroll marker pseudo-elements (new spec)
- `oklch()` color values — converted to `#rrggbbaa` + `lab()` fallbacks for broader browser compatibility
## Why?
Previously, when CSS minification encountered an error, the entire stylesheet would be marked as unparsable, losing any valid CSS that could have been recovered. This meant `turbopackIgnoreIssue` could not suppress the errors since the module produced no output regardless.
Now, when lightningcss recovers from parse errors (collecting them as warnings), issues are still emitted but processing continues with the recovered stylesheet. Only truly fatal parse errors (where `StyleSheet::parse` returns `Err`) remain non-recoverable.
The refactoring also consolidates CSS validation logic into a single function, reducing code duplication and making the error handling flow clearer.
## How?
- Created `parse_css_stylesheet()` function that wraps `StyleSheet::parse()` and applies CSS module validation
- Modified error handling in `process_content()` to call the new function and recover from minification errors by re-parsing
- Changed early returns to error emission followed by recovery attempts
- Added test fixtures and e2e tests to verify CSS parse error recovery works correctly
- Added snapshot tests showing expected output when CSS contains parse errors (valid rules are preserved, invalid ones are skipped)
- Skipped CSS parse error recovery e2e tests for webpack (webpack has its own CSS pipeline)
## Test Plan
- Added e2e tests in `css-parse-error-recovery.test.ts` that verify:
- Pages with CSS parse errors still render when using `turbopackIgnoreIssue` config
- Parse errors are properly reported in CLI output
- Added snapshot tests showing CSS output with parse errors, including modern CSS features (`::highlight`, `scroll-marker-group`, `oklch()` color conversion)
- Existing CSS tests continue to pass
