Replace manual convertor memory handling with RAII types

Replaces MEMORY_ALLOCATOR/buffer_deleter__ and the raw char * from python_scalar_to_cpp<DEV_STRING> with move-only RAII owners (net −105 lines). TangoBuffer makes the allocation strategy a template parameter, so release() (freed with delete[]) and into_corba_sequence() (freed with freebuf) are static_assert-enforced rather than review-enforced, and the manual cleanup blocks disappear. The LONG/DOUBLE-STRING array copy-paste that caused the !1051 (merged) error-message bug collapses onto one numeric_string_array_traits, dropping enum DevVarNumericStringArray and __long_double_string_array_helper.

No behaviour change, so no news entry; the memory test now covers conversions that fail partway through a string spectrum, which is the path the new element ownership protects.

Stacked on !1052 (merged); targets convertors-refactor-a until that merges.

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