perf(logs): fast-path common UTC ISO 8601 timestamps

[djwhitt]

Summary

• Parse the common production form YYYY-MM-DDTHH:MM:SS.ffffffZ directly for years 1970 through 2099.
• Validate calendar days, leap days, hours, minutes, and seconds before computing Unix microseconds.
• Retain DateTime.from_iso8601/1 as the compatibility fallback for offsets, other precision, out-of-range years, and invalid common-shape values.
• Add generated parity, boundary, invalid-date, and fallback coverage.
• Assert that BigQuery Storage API conversion still produces UTC DateTime values and Arrow timestamp data.

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