Prenner/fix hashing images

[prenner]

Summary

as detailed here, PNGs fail to hash when passed to the /h/hash endpoint. PNG hashes work via the threatexchange CLI because it is guaranteed that the contents are flushed before hashing occurs:

with tempfile.NamedTemporaryFile("wb", delete=False) as tmp:
    logging.debug("Writing -- to %s", tmp.name)
    tmp.write(resp.content)
filename = pathlib.Path(tmp.name)

but when filename is set in the with block, this is not the case for PNGs. manually call flush to fix this

https://discuss.python.org/t/should-tempfile-namedtemporaryfile-automatically-flush-the-buffer/17388 - in python 3.13 (we are on 3.11) there is a delete_on_close that we could use but deletion isn't guaranteed

Test Plan

Confirmed that the endpoint can now hash PNGs with flush added

[Dcallies]

Does this work too?

Suggested change

	with tmp.file as temp_file: # this ensures that bytes are flushed before hashing
	temp_file.write(download_resp.content)
	tmp.write(download_resp.content)
	tmp.flush()