Nokogiri: Possible Use-After-Free when directly using `NokogirI::XML::XPathContext` beyond document lifetime

Summary

### Summary `Nokogiri::XML::XPathContext` did not keep its source document alive for garbage collection. If an `XPathContext` outlived its document and the document was collected, evaluating an XPath expression could read invalid memory and potentially segfault. This is only reachable when application code constructs an `XPathContext` directly and lets the document become unreachable while continuing to use the context. The normal `Document#xpath`, `#css`, and related search methods are not affected, and it is not triggerable by malicious document input. Nokogiri 1.19.4 makes `XPathContext` keep its source document alive for as long as the context exists. Only the CRuby implementation is affected. JRuby is not affected. ### Severity The Nokogiri maintainers have evaluated this as low severity. Reaching it requires an unusual API-usage pattern that does not arise during normal use. The application must construct an `XML::XPathContext` directly and continue using it after allowing its source document to be garbage-collected. Nokogiri 1.19.4 makes this pattern safe with no change to the public API. The context now keeps its source document alive for as long as it exists. ### Mitigation Upgrade to Nokogiri 1.19.4 or later. As a workaround, ensure the source document remains referenced for as long as any `XPathContext` created from it is in use. The standard `Document#xpath`, `#css`, and related search methods already do this and are unaffected. ### Credit This issue was responsibly reported by Zheng Yu from depthfirst.com.