refactor flux2 klein kv pipeline tests to the new mixin structure

[akshan-main]

What does this PR do?

Refactors the Flux2 Klein KV pipeline tests to the new mixin structure from #14113: a Flux2KleinKVPipelineTesterConfig plus TestFlux2KleinKVPipeline(PipelineTesterMixin) and TestFlux2KleinKVPipelineMemory(MemoryTesterMixin). Follows #14326 and completes the flux2 family.

Existing coverage is kept: test_fused_qkv_projections (using assert_tensors_close), test_image_output_shape, and test_without_image, with dummy inputs switched to output_type="pt" and assertions adjusted for the channels-first layout. The test_encode_prompt_works_in_isolation skip is kept since the pipeline cannot run with transformer=None. No caching tests are added since the old suite had none.

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