test(tracing): stabilize span capture assertions

[AlainDor]

Summary

• snapshot tracing span fields on record as well as close in test capture layers
• serialize core observability capture and rebuild tracing interest cache like librarian tests
• assert against the latest span snapshot where tests inspect final recorded fields

Verification

• cargo build --workspace
• cargo test --workspace
• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --workspace --all-features -- --test-threads=32
• exact failed tracing test repeated 50 times