Degrade gracefully when a source file cannot be parsed #1259

This library uses PHP-Parser to statically analyse every file that is covered: executable lines, code units, ignored lines, dead code, complexity, and size metrics. When PHP-Parser cannot parse a file, ParsingSourceAnalyser wraps the parse error in a ParserException.

This exception is not recoverable for the consumer. In PHPUnit, static analysis runs as part of CodeCoverage::append(), so every test whose coverage touches the unparseable file errors with "Cannot parse ...", and report generation fails for every configured format as well. A single unparseable file effectively disables code coverage for the whole test run.

This is about to become a practical problem (again): PHP 8.6 introduces partial function application, and until PHP-Parser supports that syntax, any covered file that uses it cannot be parsed. The same situation will recur with every future PHP version whose syntax additions land before PHP-Parser support does.

Proposal

Treat a parse failure as a per-file degradation instead of a fatal error:

• ParsingSourceAnalyser catches the parser error and returns an AnalysisResult that is marked as "could not be parsed" and carries the parser's error message. The degraded result has no code units, no executable/dead/branch-operator lines, and line counts without the comment/non-comment split. Lines ignored using line-based annotations (@codeCoverageIgnore comments) are still honored, because that scan is based on token_get_all() and does not require the parser.
• FilterProcessor::applyExecutableLinesFilter() skips files whose analysis is degraded, leaving the raw data reported by the driver untouched. Line coverage for such a file is then based on the driver's notion of executable lines, which is what php-code-coverage did for all files before static analysis was introduced.
• The degradation must not be silent: CodeCoverage::parseErrors() exposes the affected files together with the parser's error message so that a test runner can warn about them once per run.
• CachingSourceAnalyser does not write degraded results to the static analysis cache. The cache key does not include the installed PHP-Parser version, so a cached degraded result would outlive a PHP-Parser upgrade that adds support for the syntax in question.

A file that PHP-Parser cannot parse gets approximate, driver-based line coverage and appears in reports without class/method/complexity details, accompanied by a warning. All other files are unaffected. Tests no longer error and reports are still generated.