test(runtime): expand json module unit tests
The hand-rolled JsonValue parser/renderer in runtime had only two
tests covering a self-contained JSON parser, renderer, accessor set,
and error type. Add focused unit tests covering primitive rendering,
sorted object keys, escape handling (quotes, backslashes, control
chars), signed-integer and whitespace-tolerant parsing, nested
structures, render->parse round-tripping, malformed-input rejection,
i64 overflow rejection, accessor variant mismatches, and JsonError
Display.
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