fix: guard against ArrayIndexOutOfBounds in readFieldName whitespace skip

[waterWang]

Fix

Adds bounds checks to the two whitespace-advancement loops in JSONReaderASCII.readFieldName() that could read past the array end when the input ends with whitespace after a field name (or after the colon).

// Before: no bounds check
while (c <= ' ' && ((1L << c) & SPACE) != 0) {
    offset++;
    c = bytes[offset];  // ArrayIndexOutOfBounds when offset == end
}

// After: bounds check, set EOI at end
while (c <= ' ' && ((1L << c) & SPACE) != 0) {
    c = offset + 1 >= end ? EOI : bytes[++offset];
}

JSON.parse("{\"a\" ") now correctly throws JSONException instead of ArrayIndexOutOfBoundsException.

Fixes #7826

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