test(e2e): stabilize invalidation slots in proposer invalidates multiple checkpoints

[spalladino]

Summary:

• Scan for consecutive bad checkpoint slots whose prior pipelined target slot is not owned by either intended bad proposer.
• Keep the malicious-config injection tied to the selected bad proposers and remove the now-unnecessary non-null assertion.
• Add an inline comment documenting why the prior pipelined target slot matters.

Why:
The test applies malicious checkpoint config while sequencers are already running. With proposer pipelining, the previous target slot can snapshot that config before the intended bad slots are built. If that prior proposer is one of the intended bad proposers, the test may spend the malicious config on the wrong checkpoint and stop validating the intended two-checkpoint invalidation path. This mirrors the slot-selection issue fixed for the invalid proposal slashing test, but applies it to the consecutive checkpoint invalidation scenario.

Testing:

• yarn format end-to-end
• yarn build
• LOG_LEVEL="info; debug:sequencer,publisher,validator" yarn workspace @aztec/end-to-end test:e2e e2e_epochs/epochs_invalidate_block.parallel.test.ts -t "proposer invalidates multiple checkpoints"

[AztecBot]

Flakey Tests

🤖 says: This CI run detected 1 tests that failed, but were tolerated due to a .test_patterns.yml entry.

\033FLAKED\033 (8;;http://ci.aztec-labs.com/b7639f5b8e4d5e93�b7639f5b8e4d5e938;;�):  yarn-project/end-to-end/scripts/run_test.sh simple src/e2e_epochs/epochs_invalidate_block.parallel.test.ts "archiver skips a descendant of an invalid-attestations checkpoint" (397s) (code: 0) group:e2e-p2p-epoch-flakes