Fix convertor memory leaks and DevVarDoubleStringArray error type

Fixes three memory leaks in ext/convertors: the string elements of spectrum/image write values, empty array attributes extracted as bytes/bytearray/str, and the scalar DevString write value. Also corrects fast_convert2array<DEVVAR_DOUBLESTRINGARRAY>, which reported malformed input as a long-string error. tests/valgrind/memory_test.py is extended to reach WAttribute.set_write_value and binary extraction of empty arrays — neither was covered before, and that is how the scalar leak surfaced; all three leaks are confirmed gone under valgrind.

Closes #782 (closed), #783 (closed), #784 (closed)

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