test: cover cooldown boundary

[anhtnt90dev]

Fixes #3

Summary

• Add a consensus test for the exact cooldown_sec boundary
• Document the current boundary behavior: repeats are suppressed only while elapsed time is strictly less than cooldown_sec
• Use integer timestamps in the boundary case to avoid floating-point noise around the exact limit

Test plan

• uv run --with pytest python -m pytest 6_Server/test_consensus.py -q
• uv run --with pytest python -m pytest -q
• git diff --check