Skip stale compaction progress metrics

[matthewsanetra]

Summary

• treat missing compaction source state as stale instead of panicking
• skip progress metrics for stale compactions and emit a warning
• add focused coverage for target-segment accounting and missing sources

Why

A compaction can remain in the active set after its source state has disappeared from the current manifest. Metrics collection previously assumed that state was always present and could panic while calculating progress.

Impact

Compaction continues safely when metrics encounter stale work; valid active compactions retain their existing progress accounting.

Validation

• cargo fmt --all -- --check
• cargo test --package slatedb test_calculate_estimated_source_bytes

cargo clippy --package slatedb --lib -- -D warnings remains blocked by pre-existing warnings elsewhere in SlateDB; the focused tests pass.