refactor bria fibo pipeline tests to the new mixin structure

[akshan-main]

What does this PR do?

Refactors the Bria FIBO pipeline tests to the new mixin structure from #14113: a BriaFiboPipelineTesterConfig plus TestBriaFiboPipeline(PipelineTesterMixin) and TestBriaFiboPipelineMemory(PipelineOffloadTesterMixin). The memory class uses the offload mixin only, since the old file disabled the layerwise casting and group offloading tests.

Existing coverage is kept: test_bria_fibo_different_prompts and test_image_output_shape, with dummy inputs switched to output_type="pt" and a fresh CPU slice in test_inference. The test_encode_prompt_works_in_isolation skip is carried over with its original reason. Follows #14547.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline?
• Did you read our philosophy doc (important for complex PRs)?
• Was this discussed/approved via a GitHub issue or the forum? Discussed on Slack with @sayakpaul
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

Who can review?

@sayakpaul

[sayakpaul]

/diffusers-bot pytest tests/pipelines/bria_fibo/test_pipeline_bria_fibo.py

[github-actions]

✅ pytest tests/pipelines/bria_fibo/test_pipeline_bria_fibo.py passed on GPU — view logs.