test: add unit tests for health module (#334)

[Wirasm]

Summary

• Health module had 537 lines across 3 files with zero test coverage
• Added 24 unit tests covering all pure logic and filesystem persistence functions
• Refactored storage functions to accept &Path for testability (public API unchanged)

Changes

File	Change
crates/kild-core/src/health/operations.rs	Added 14 tests: calculate_health_status (9), enrich_session_with_health (3), aggregate_health_stats (2)
crates/kild-core/src/health/storage.rs	Added save_snapshot_to, load_history_from, cleanup_old_history_in internal variants + 10 tests for snapshot conversion, round-trip storage, cleanup, and date filtering

Testing

• cargo test -p kild-core -- health — 24 new tests pass
• cargo test --all — 146 tests pass, no regressions
• cargo clippy --all -- -D warnings — clean
• cargo fmt --check — clean

Validation

cargo test -p kild-core -- health && cargo clippy --all -- -D warnings && cargo fmt --check

Issue

Fixes #334

[Wirasm]

Self Code Review

Summary

Solid test-only PR that adds 24 unit tests covering all business logic in the health module (operations.rs + storage.rs). No behavioral changes.

Findings

Strengths

• Covers all 3 public functions in operations.rs and all storage operations in storage.rs
• Good edge case coverage: corrupted files, nonexistent dirs, empty inputs, boundary conditions
• Clean refactoring: save_snapshot_to/load_history_from/cleanup_old_history_in variants follow established testability pattern
• Uses tempfile::TempDir for filesystem isolation and Session::new_for_test helper
• Time-based tests use 200min offsets to avoid races with parallel test execution on global AtomicU64 threshold

Scope notes (non-blocking)

• handler.rs (94 lines) is intentionally not tested — it's pure orchestration calling sessions::handler, process, and config, which would require integration-level mocking for minimal value

Security

• No security concerns — test-only changes with no production behavior changes

Checklist

• Fix addresses root cause (zero test coverage → 24 tests)
• Code follows codebase patterns
• Tests cover the change
• No bugs introduced (no production code behavior changes)