Fix restype of CancellableSendMessage (#19862)
May fix #19729
### Summary of the issue:
* as defined in `nvdaHelper/local/nvdaHelperLocal.cpp`, the return type
of `cancellableSendMessageTimeout` is `LRESULT`.
* `LRESULT` is defined as `__int64` on 64-bit Windows, and `long` on
32-bit Windows. That is, `LRESULT` is the signed machine-word-sized
integer type.
* However, in `source/NVDAHelper/localLib.py`, its return type was set
to `c_int`. That is, the signed 32-bit integer type.
* This may be a contributing factor to #19729, as `ScintillaTextInfo`
uses `cancellableSendMessageTimeout` when getting the text offset from
point. Since the the return type is signed, very large (but still
positive 64-bit two's compliment) integers may be interpreted as
negative due to being truncated.
* On the other hand, if my math is correct, this would only start
manifesting in files that are 2GiB or larger, though the reporters did
say the files were large.
### Description of user facing changes:
Unknown, but may fix a crash in Notepad++ (and potentially other cases)
### Description of developer facing changes:
`NVDAHelper.localLib.cancellableSendMessageTimeout.restype` is now
`c_longlong`
### Description of development approach:
Defined `LRESULT` as `c_ssize_t` (`c_long` on 32-bit; `c_longlong` on
64-bit), and set `NVDAHelper.localLib.cancellableSendMessageTimeout`'s
`restype` to `LRESULT`.
### Testing strategy:
Ran from source and did normal work.
I don't think this has the potential to cause issues, unless we
introduced code relying on the incorrect restype during the 64-bit
migration, as the return type was fine on 32-bit builds (`c_int is
c_long`).
### Known issues with pull request:
None
