test(audit): add coverage for audit log recording and pagination (#747)

[sammajayi]

Summary

Added a #[cfg(test)] module to audit.rs with 24 tests covering audit log recording and pagination correctness.

Related Issue

Closes PhasoraLabs#747

Changes Made

• AuditAction variant coverage (13 tests): one test per variant verifying action, actor, milestone_idx, and amount fields
• Pagination correctness (5 tests): get_recent with empty log, zero n, n > len, exact n == len, and oldest-first slicing
• Append-only invariant (3 tests): monotonically growing length, no overwrites, order preserved across grants
• Metadata and helpers (3 tests): grant_id, timestamp/ledger_sequence, different actors, log_length consistency

Testing

• cargo check --lib passes with no new warnings
• cargo fmt --check clean
• cargo test blocked by pre-existing compilation errors in reviewer_sla.rs on main

Notes for Reviewer

The test suite has pre-existing compilation errors in reviewer_sla.rs that prevent cargo test from running. These exist on main as well.