fix: SAX::ParserContext keeps a reference to the input

[flavorjones]

What problem is this PR intended to solve?

I saw a failure in CI at https://github.com/sparklemotion/nokogiri/actions/runs/12529532790/job/34945362057?pr=3393

  1) Error:
Nokogiri::HTML4::SAX::ParserContext::constructor::encoding::.io#test_0001_supports passing encoding name:
NotImplementedError: method `read' called on terminated object (0x0000005520809790)
    test/html4/sax/test_parser_context.rb:60:in `parse_with'
    test/html4/sax/test_parser_context.rb:60:in `block (5 levels) in <module:SAX>'

So now the class keeps a reference to the input object to prevent it from being GCed before we parse it.

Have you included adequate test coverage?

Yes.

Does this change affect the behavior of either the C or the Java implementations?

N/A