test(gallery): extend importer specs to cover buun-llama-cpp

Two additions that pair with the new backend:
- An Import()-side case that asserts preference buun-llama-cpp produces
  backend: buun-llama-cpp in the emitted YAML (mirrors the existing
  ik-llama-cpp and turboquant cases).
- AdditionalBackends() spec now asserts all three drop-in replacements
  are advertised, and verifies buun-llama-cpp's Modality/Description
  alongside the other two.

Assisted-by: Claude:Opus-4.7 [Read] [Edit] [Bash]
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>
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