feat(dllm): diffusiongemma gallery entry and e2e coverage

Gallery model diffusiongemma-26b-a4b-it (unsloth BF16 GGUF, sha256
verified against the HF LFS oid) with use_tokenizer_template and an
honest experimental/throughput description. e2e: BACKEND_BINARY-mode
specs boot the real gRPC backend with the tiny fixture model (templated
chat + streaming); real-26B specs are separately env-gated. Adds an
opt-in BACKEND_TEST_SEED knob so random-weight fixture models run the
generic specs deterministically.

Assisted-by: Claude Code (Fable 5)
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>
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