fix(gallery/ltx-2.3): add vae_decode_only:false for i2v / flf2v (#9987)

LTX-2.3 i2v inference fails inside generate_video with:

  [ERROR] LTXAV image conditioning requires VAE encoder weights;
  create the context with vae_decode_only=false

Without vae_decode_only:false in the options block, gosd.cpp creates
the sd_ctx with VAE encoder weights freed, so latent encoding of the
init_image is impossible. Adding the option mirrors what we already
do for Wan i2v entries.

Affects all six LTX-2.3 entries (dev/distilled × UD-Q4_K_M, Q4_K_M,
Q8_0). T2V wasn't impacted by the missing option since it has no
init image to encode, which is why the T2V smoke earlier passed.

Assisted-by: Claude:claude-opus-4-7