perf(logs): reuse clean BigQuery-safe payloads

[djwhitt]

Summary

• Scan nested maps and lists for empty values and BigQuery-unsafe keys before rebuilding them.
• Return the original payload when cleaning and key normalization would make no changes.
• Retain the fused cleaning traversal and collision fallback for payloads that require normalization.
• Extend coverage for clean nested payloads and nested slow-path cases.

Stack

1. perf(logs): reuse clean BigQuery-safe payloads #3765 — Reuse clean BigQuery-safe payloads
2. perf(logs): transform bodies before constructing LogEvent #3767 — Transform bodies before constructing LogEvent
3. perf(logs): normalize integer timestamps without digit lists #3768 — Normalize integer timestamps without digit lists
4. perf(logs): fast-path common UTC ISO 8601 timestamps #3769 — Fast-path common UTC ISO 8601 timestamps