fix(monitors): Fix is_upserting flag reset logic

[evanpurkhiser]

The is_upserting flag was being incorrectly reset when a completing
check-in without config was updating an existing IN_PROGRESS check-in.

The bug was that the flag was being reset too early in the flow, before
we knew whether we were creating a new check-in or updating an existing one.

Changes:

• Removed premature is_upserting reset from _ensure_monitor_with_config()
• Moved reset logic to after check-in creation, where we know if we created
  a brand new check-in vs updating an existing one
• Renamed monitor_config to checkin_monitor_config in the check-in creation
  code to avoid variable shadowing and allow referencing the original
  monitor_config from params

The flag should only be reset to False when:

1. A brand new check-in is created (not updating an existing one)
2. No monitor config was provided in the check-in params
3. The monitor was previously marked as is_upserting=True

Fixes NEW-584: is_upserting prematurely reset when updating a in_progress

[linear]

NEW-584 `is_upserting` prematurely reset when updating a `in_progress`

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #101556      +/-   ##
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+ Coverage   79.33%    80.97%   +1.64%     
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  Files        8704      8706       +2     
  Lines      386898    386933      +35     
  Branches    24520     24520              
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+ Hits       306960    313336    +6376     
+ Misses      79587     73246    -6341     
  Partials      351       351