tests: pin generation order of invalid-ER fraud proofs (#1892)

[vedhavyas]

Summary

Adds six integration tests in domains/client/domain-operator/src/tests.rs
that pin the priority order of DomainBlockProcessor::generate_fraud_proof().
Each test corrupts a single ER in the priority-N field plus everything
below it on the same receipt, submits the malicious bundle, and asserts
the matching FraudProofVariant fires — confirming higher-priority
mismatches win when multiple fields are simultaneously bad.

Existing single-field FP tests cover each variant in isolation; this PR
fills the only remaining gap (ordering when multiple fields are wrong).

Closes #1892.

I recommend reviewing commit-by-commit

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