chore: remove unused ATTR_START_TIME, ATTR_END_TIME, and ATTR_SESSION_OPTIONS telemetry constants

[detail-app]

Remove three dead telemetry span attribute constants from
trace_types.py along with their commented-out call sites in
agent_session.py and stale SAFE_KEYS entries in test_trace_types_pii.py.

• ATTR_START_TIME / ATTR_END_TIME: their active set_attribute call sites
  in agent_session.py were commented out in favor of OpenTelemetry's
  native span start_time/end_time parameters, leaving the constants
  with zero production references (only commented-out lines).
• ATTR_SESSION_OPTIONS: its sole call site was removed when the session
  span setup was refactored; the constant had no remaining references.

History

• Last materially changed October 2025 (add participant attributes to traces #3725 @theomonnom)

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.