Raise on non-UTF-8 string keys in RFC 8785 mode

[Shaisolaris]

Follow-up to #1108. As suggested there, string keys that are not UTF-8 are treated as invalid in RFC 8785 mode, so the generator now raises instead of transcoding them.

On master:

latin1 = "\xE9".dup.force_encoding(Encoding::ISO_8859_1)
JSON.generate({ latin1 => 1, "é" => 2 }, rfc8785: true)
# => "{\"é\":1,\"é\":2}"   (duplicate key in canonical output)
JSON.generate({ "\xC3\xA9".b => 1 }, rfc8785: true)
# => Encoding::UndefinedConversionError (from the key sort, not JSON::GeneratorError)
JSON.generate({ "\xFF".dup.force_encoding("UTF-8") => 1 }, rfc8785: true)
# => Encoding::InvalidByteSequenceError

With this change, all three raise JSON::GeneratorError. The checks are in rfc8785_sort_keys_proc, on the path where no as_json conversion is used:

• ASCII-only keys are still accepted in any ASCII-compatible encoding (for example "a".b, US-ASCII strings, and Symbols).
• Keys in encodings that are not ASCII-compatible, such as UTF-16LE, are rejected even when their content is ASCII. Before, { "a".encode("UTF-16LE") => 1 } produced {"a":1}.
• Keys that go through as_json (with strict: true) are converted as before.
• Only rfc8785: true is affected. Plain JSON.generate keeps its current behavior.

Tests:

• Added test_rfc8785_non_utf8_string_keys. It fails on master and passes with this change.
• All test/json/*_test.rb files pass locally with the C extension on Ruby 3.3.8, with the same counts as master apart from the new test. I did not run the TruffleRuby or JRuby generators or the CI matrix.