Fix image upload processing and img2img pipeline in diffusers backend (#8879)

* fix: add missing bufio.Flush in processImageFile

The processImageFile function writes decoded image data (from base64
or URL download) through a bufio.NewWriter but never calls Flush()
before closing the underlying file. Since bufio's default buffer is
4096 bytes, small images produce 0-byte files and large images are
truncated — causing PIL to fail with "cannot identify image file".

This breaks all image input paths: file, files, and ref_images
parameters in /v1/images/generations, making img2img, inpainting,
and reference image features non-functional.

Signed-off-by: Attila Györffy <attila+git@attilagyorffy.com>

* fix: merge options into kwargs in diffusers GenerateImage

The GenerateImage method builds a local `options` dict containing the
source image (PIL), negative_prompt, and num_inference_steps, but
never merges it into `kwargs` before calling self.pipe(**kwargs).
This causes img2img to fail with "Input is in incorrect format"
because the pipeline never receives the image parameter.

Signed-off-by: Attila Györffy <attila+git@attilagyorffy.com>

* test: add unit test for processImageFile base64 decoding

Verifies that a base64-encoded PNG survives the write path
(encode → decode → bufio.Write → Flush → file on disk) with
byte-for-byte fidelity. The test image is small enough to fit
entirely in bufio's 4096-byte buffer, which is the exact scenario
where the missing Flush() produced a 0-byte file.

Also tests that invalid base64 input is handled gracefully.

Signed-off-by: Attila Györffy <attila+git@attilagyorffy.com>

* test: verify GenerateImage merges options into pipeline kwargs

Mocks the diffusers pipeline and calls GenerateImage with a source
image and negative prompt. Asserts that the pipeline receives the
image, negative_prompt, and num_inference_steps via kwargs — the
exact parameters that were silently dropped before the fix.

Signed-off-by: Attila Györffy <attila+git@attilagyorffy.com>

* fix: move kwargs.update(options) earlier in GenerateImage

Move the options merge right after self.options merge (L742) so that
image, negative_prompt, and num_inference_steps are available to all
downstream code paths including img2vid and txt2vid.

Signed-off-by: Attila Györffy <attila+git@attilagyorffy.com>

* test: convert processImageFile tests to ginkgo

Replace standard testing with ginkgo/gomega to be consistent with
the rest of the test suites in the project.

Signed-off-by: Attila Györffy <attila+git@attilagyorffy.com>

---------

Signed-off-by: Attila Györffy <attila+git@attilagyorffy.com>
Co-authored-by: Ettore Di Giacinto <mudler@users.noreply.github.com>

backend/python/diffusers/backend.py

@@ -741,6 +741,8 @@ class BackendServicer(backend_pb2_grpc.BackendServicer):
         # populate kwargs from self.options.
         kwargs.update(self.options)
 
+        kwargs.update(options)
+
         # Set seed
         if request.seed > 0:
             kwargs["generator"] = torch.Generator(device=self.device).manual_seed(