Add not_params to ModularPipelineTesterMixin

[yiyixuxu]

Pipelines sometimes deliberately drop a canonical param — e.g. guidance-distilled pipelines don't accept negative_prompt. Test classes can now declare these in not_params and test_pipeline_call_signature asserts they are absent from the pipeline's inputs, so reintroducing one by accident fails the test.

Adopted in the flux2-klein test classes (negative_prompt).

What does this PR do?

Fixes # (issue)

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[yiyixuxu]

for #14177 (comment)