Feed the ENOSPC test the pixel format it declares

DetJF4M is signed - a JUNGFRAU in photon-counting conversion is signed by default - and the fixture
handed the writer uint16 images, so the pixel-format cross-check added in this branch refused them
and the test failed before it reached what it is about. Nothing here reads a pixel value back.

Missed when the other fixtures were corrected, because jfjoch_hdf5_enospc_test is a separate binary
that jfjoch_test does not run; CI runs it as its own step.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VfYvJT5Nb71suJCowRBn5z

tests/jfjoch_hdf5_enospc_test.cpp

@@ -33,7 +33,10 @@ TEST_CASE("FileWriter_enospc") {
         x.FillMessage(start_message);
 
         FileWriter file_set(start_message);
-        std::vector<uint16_t> image(x.GetPixelsNum());
+        // int16, because a JUNGFRAU in photon-counting conversion is signed by default and the
+        // writer refuses a stream whose images do not match the format its start message declares.
+        // Nothing here reads a pixel value back - the test is about running out of space.
+        std::vector<int16_t> image(x.GetPixelsNum());
 
         DataMessage message{};
         message.image = CompressedImage(image,  x.GetXPixelsNum(), x.GetYPixelsNum());