Test coverage

[CollinsC1O]

Summary

• Fix crate-breaking merge corruption (mismatched braces, duplicate function/enum names, interleaved event structs) that had left hello-world unable to compile at all — none of the existing tests had ever actually run.
• Add tests/reputation_test.rs (12 new tests): full contract-level coverage of sender reputation tracking (record_sender_delivery_success/failure, get_sender_reputation*), which previously had zero test coverage.
• Wire the orphaned tests/batch_event_test.rs into the build — it existed on disk but was never included in lib.rs's test module tree.
• Fix two real bugs surfaced by getting the suite running: a reputation-score formula that could never exceed 75 (Gold/Platinum tiers were unreachable), and an off-by-one in a test helper reading the wrong event topic (category vs. priority).
• De-flake a gas-benchmark test in batch_ack_test.rs that relied on non-monotonic CPU-instruction metering; replaced with a deterministic invocation-count + functional-equivalence assertion.

Test plan

• cargo test -p hello-world — 315/315 passing (0 before, since the crate didn't compile)
• cargo build -p hello-world — clean build

Closes #403

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