[Nexthop][fboss2-dev] ConfigSession: always create initial commit on fresh /etc/coop

[vybhav-nexthop]

Pre-submission checklist

• I've ran the linters locally and fixed lint errors related to the files I modified in this PR. You can install the linters by running pip install -r requirements-dev.txt && pre-commit install
• pre-commit run

##Summary

• On a fresh box with no git history, initializeGit() only created
  an initial commit if cli/agent.conf already existed. On first-ever
  use that file doesn't exist yet, so concurrent users end up with
  base_ = "".
• When User1 commits and HEAD moves, User2's stale-session check
  (!base_.empty() && currentHead != base_) short-circuits on the empty
  string, letting User2 silently overwrite User1's changes.
• Fix: always create an initial commit when the repo has no commits.
  Commit agent.conf if it exists (as a copy — we can't rename because
  /etc/coop/agent.conf must stay in place; commit() replaces it
  with a symlink later), falling back to an empty {} JSON object. A
  try/catch handles the concurrent-init race.

Test plan

• Run ConfigConcurrentSessionsTest.ConflictAndRebase on a fresh
  box (no prior git history in /etc/coop) — should now fail User2's
  commit with "system configuration has changed"
• Confirm existing ConfigSession unit/integration tests still
  pass