apollo_consensus_orchestrator: test transactions with proofs in validate_proof

[einat-starkware]

Note

Low Risk
Test-only changes that expand coverage for proof-verification failure handling without modifying production validation logic.

Overview
Adds a new validate_proposal regression test that simulates client-side proof verification failures by returning a VerifyAndStoreProofTask that errors, and asserts the validator aborts the proposal and surfaces ValidateProposalError::ProposalPartFailed with the verification message.

Minor test cleanup removes outdated TODO comments in test_utils around client-side proving/verification handles.

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[einat-starkware]

• apollo_consensus_orchestrator: test transactions with proofs in validate_proof #12367 👈 (View in Graphite)
• apollo_consensus_orchestrator: make proof verification happen asynchronously with tx execution #12240
• apollo_transaction_converter: return a verification handle when converting transactions #12239 : 1 other dependent PR (#12288 )
• apollo_transaction_converter: create verification handler and spawn verification #12238
• main-v0.14.2

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