fix: grow strBuf when reading long escaped strings, for issue #7671

[kamilkrzywanski]

What this PR does / why we need it?

Keep JSONReaderUTF16.strBuf in sync when the local buffer is grown mid-loop while reading escaped strings.

Issue #7671 reported ArrayIndexOutOfBoundsException on 2.0.61 when a long escaped string followed a shorter one that had already initialized strBuf (e.g. char[512]). The live crash path is already fixed on main by #3989 (else if (stroff > strBuf.length) before the initial arraycopy, released in 2.0.62). This change is a buffer-reuse / consistency follow-up: when strBuf is resized inside the escape loop, write the grown array back to this.strBuf so later reads reuse the larger buffer instead of re-growing from the stale smaller one.

Summary of your change

• Always assign this.strBuf after initial allocate/grow, and after mid-loop growth in JSONReaderUTF16.readString.
• No dedicated regression test: [BUG] java.lang.ArrayIndexOutOfBoundsException: arraycopy: last destination index 6232 out of bounds for char[576] #3989 / Issue3989 already covers the AIOOBE case; a black-box parse test would not fail if these mid-loop assignments were reverted.

Please indicate you've done the following:

• Made sure tests are passing and test coverage is added if needed.
• Made sure commit message follow the rule of Conventional Commits specification.
• Considered the docs impact and opened a new docs issue or PR with docs changes if needed.

[wenshao]

Reviewed — no blockers. The change is correct and correctly scoped: the hoisted and mid-loop this.strBuf = strBuf assignments are equivalent/safe, and JSONReaderUTF16 is the only reader with this reusable-buffer pattern (JSONReaderUTF8 writes the grown buffer back to its thread-local cache; the ASCII/Str paths don't use strBuf). Build and the new Issue7671 tests pass (verified locally on JDK 17). One non-blocking suggestion inline.

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[wenshao]

[Suggestion] These regression tests don't gate the source change — both pass identically with the two mid-loop this.strBuf = strBuf; assignments reverted. Verified empirically: reverting only the source edits and running Issue7671 gives Tests run: 2, Failures: 0 (test() and testStrBufGrow() both green). The pre-existing else if (stroff > strBuf.length) guard (added in commit bdfe92f for issue #3989) already prevents the original ArrayIndexOutOfBoundsException at the initial arraycopy, and the parsed result is always built from the correctly-grown local strBuf; so on current main this is a buffer-reuse/consistency improvement rather than a live crash fix. In testStrBufGrow the two added lines never even execute: key "b" has a leading run stroff=600 > 512, so the else if grows the buffer, and the total 651 < newCapacity(600,512)=768, so neither mid-loop guard fires. — Concrete cost: a future revert of the two lines would not be caught by these tests. Consider a white-box assertion (e.g. via reflection) that strBuf capacity grows after parsing consecutive long escaped strings, or a short note that this is a reuse/consistency follow-up to #3989 rather than a new crash fix. (The getBytes(UTF_8) path uses JSONReaderUTF8, which writes the grown buffer back to its cache, so it doesn't exercise the changed code.)

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