fix(flux2): strip reference-image tokens from Heun preview latents (invoke-ai#9274)

The Heun (2nd order) branch of the FLUX.2 Klein denoise loop passed the
full preview latent to step_callback without removing the concatenated
reference-image tokens. With ref images the sequence length is doubled,
so unpack_flux2 failed with "Shape mismatch, 8192 != 4096".

Slice the preview to original_seq_len before the callback, matching the
behavior already present in the Euler branch.

Co-authored-by: Lincoln Stein <lincoln.stein@gmail.com>

Author: Pfannkuchensack

invokeai/backend/flux2/denoise.py

@@ -195,13 +195,17 @@ def denoise(
                             preview_img = inpaint_extension.merge_intermediate_latents_with_init_latents(
                                 preview_img, 0.0
                             )
+                        # Extract only the generated image portion for preview (exclude reference images)
+                        callback_latents = (
+                            preview_img[:, :original_seq_len, :] if img_cond_seq is not None else preview_img
+                        )
                         step_callback(
                             PipelineIntermediateState(
                                 step=user_step,
                                 order=2,
                                 total_steps=total_steps,
                                 timestep=int(t_curr * 1000),
-                                latents=preview_img,
+                                latents=callback_latents,
                             ),
                         )
             else: