[smile] Optimize async short ASCII string decoding

[Dongnyoung]

Summary

This PR optimizes contiguous short-ASCII VALUE_STRING decoding in the Smile async parser.

NonBlockingByteArrayParser previously decoded these values by copying bytes into TextBuffer's char buffer and then materializing a String. When the complete short ASCII value is already available in the input buffer, this PR constructs the String directly from the input bytes using StandardCharsets.US_ASCII and resets the TextBuffer with that string.

This mirrors the approach already used by the synchronous Smile parser for the equivalent contiguous short-ASCII path.

Changes

• Use direct byte[] to String decoding for contiguous short ASCII values in NonBlockingByteArrayParser
• Preserve TextBuffer state with resetWithString(str)
• Add async parser coverage for short ASCII string accessors with contiguous input

Benchmark

The benchmark exercises the actual async NonBlockingByteArrayParser path on JDK 21.0.7 using JMH 1.37.

Setup:

• 5 forks
• 5 × 1 s warmup
• 8 × 1 s measurement
• 4096 unique ASCII values per invocation
• Complete Smile input supplied in one ByteArrayFeeder.feedInput() call
• CHECK_SHARED_STRING_VALUES disabled
• Lengths: 8, 32, and 64 (SmileConstants.MAX_SHORT_VALUE_STRING_BYTES)
• GC profiler enabled

JsonParser.getString()

Length	Base ns/value	Candidate ns/value	Reduction	Base B/op	Candidate B/op
8	13.661 ± 0.326	12.854 ± 0.278	5.9%	48.156	48.156
32	17.785 ± 0.580	12.630 ± 0.211	29.0%	72.156	72.156
64	23.674 ± 0.411	13.845 ± 0.374	41.5%	104.156	104.156

JsonParser.getStringCharacters()

Length	Base ns/value	Candidate ns/value	Reduction	Base B/op	Candidate B/op
8	16.890 ± 0.727	14.379 ± 0.153	14.9%	80.156	80.156
32	21.764 ± 0.574	16.010 ± 0.136	26.4%	152.156	152.156
64	29.892 ± 1.058	19.436 ± 0.134	35.0%	248.156	248.156

Normalized allocation was effectively unchanged. The measured improvement therefore appears to come primarily from avoiding the intermediate byte-to-char decoding/copying step rather than from reducing allocation.

Testing

./mvnw -pl smile -Dtest=SimpleStringArrayTest -Dsurefire.useModulePath=false test