Make GeneratorState#_configure only write the options it is given

[youdie006]

Follow-up to #1076, which aligned the pure generator and left the Java one, marking the gap with two omit ... if RUBY_ENGINE == 'jruby' lines. This closes it and re-enables both tests.

GeneratorState#_configure re-wrote every option with its class default, so any configure/merge reset the options it was not given:

state = JSON.state.new(max_nesting: 3, allow_nan: true, ascii_only: true, script_safe: true)
state.configure(indent: '1')
# JRuby: max_nesting 100, allow_nan false, ascii_only false, script_safe false
# C ext and pure generator: 3, true, true, true

The allow_nan reset is the sharpest — a state that generated NaN a moment earlier starts raising JSON::GeneratorError. Through the public output, JSON.state.new(sort_keys: true) after any configure gave {"b":1,"a":2} on JRuby and {"a":2,"b":1} elsewhere. A string option given as an explicit nil had the mirror problem, keeping the previous value instead of clearing.

configure_state_i (generator.c:1860) is an if/else if chain over rb_hash_foreach, so it writes only the keys present. The non-string options now take the current field as their default, which is the minimal change that makes "absent" mean "leave alone":

maxNesting = opts.getInt("max_nesting", maxNesting);

OptionsReader.get delegates to RubyHash#delete: absent gives Java null (keep current), present-but-nil gives nil (falsy), which is what long_config/RTEST do on the C side. as_json and sort_keys need an explicit hasKey, and the five string options need a small helper because getString returns null for absent and falsy. hasKey does not delete, so ensureEmpty() still rejects unknown keywords.

Verification

Built with rake create_jar in jruby:9.4-jdk21, every row rebuilt from scratch, each row's GeneratorState.java md5 distinct.

row	result
the two re-enabled tests on master	4 failures
with the fix	8 tests, 35 assertions, 0 failures
helper drops its hasKey guard	3 failures
sort_keys unguarded	1 failure
max_nesting back to DEFAULT_MAX_NESTING	1 failure
as_json unguarded	1 failure

The as_json row is why there is a third test: with only the two re-enabled tests that mutant survived, so the guard would have shipped uncovered. I checked the new assertion against the C extension first — it passes there unchanged.

engine	result
JRuby 9.4.15.0	589 tests, 3000 assertions, 0 failures
CRuby 3.2.11, C extension	595 tests, 3423 assertions, 0 failures

Java is not compiled on CRuby and the two omits were jruby-guarded, so those rows already ran these tests. TruffleRuby loads lib/json/truffle_ruby/generator.rb, which #1076 already fixed; I did not run that engine.

Not measured: hasKey adds a second hash lookup for as_json/sort_keys. configure is not the generate hot path, but I have not benchmarked it.

If you would prefer the C shape here — iterate the given hash once and switch on the key — I am happy to redo it that way; it is larger because ensureEmpty relies on get deleting as it consumes.

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Disclosure: I used Claude (an AI assistant) while preparing this change, as on #1076. Every result above I ran and verified myself.