The test incorrectly suggested the plug handles duplicate headers,
but put_req_header replaces values. Cloudflare only sends one
cf-timezone header, making this test unnecessary.
- Update AgentAuth plug to use synchronous heartbeat updates in test env
- Refactor async heartbeat logic to reduce nesting (Credo)
- Remove Process.sleep from tests (no longer needed with sync updates)
This fixes 'owner exited' database errors in the test suite by avoiding
async Task spawns during tests, which caused DB connection ownership
conflicts with Ecto's sandbox mode.
