feat(crons): Report "true" transition timestamp in DecisionResult

[evanpurkhiser]

When transition into and out of an incident we want to know the true start time and true recovery time of the incident. We can do this with the following logic:

• When starting an incident we backfill the ABNORMAL decisions to INCIDENT. Record the timestamp of the very first abnormal and report that back with the DecisionResult that is transition=INCIDENT_STARTED

• When recovering from an incident we backfill the RECOVERING decisions as NORMAL. Record the timestamp of the very first normal decision and report that back with the DecisionResult that is transition=INCIDENT_RECOVERED

For all other decisions (including abnormal recovery and incident recovery failed) the timestamp is always just the tick that we're making the decision for, since these do not have

The resulting time range is an inclusive start and exclusive end to determine the datetime range of the incident.

Part of GH-79328

[JoshFerge]

looks good!

[codecov]

Codecov Report

Attention: Patch coverage is 95.65217% with 1 line in your changes missing coverage. Please review.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/sentry/monitors/system_incidents.py	95.65%	0 Missing and 1 partial ⚠️

Additional details and impacted files

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  Branches    44004    44005       +1     
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+ Hits       250716   250725       +9     
- Misses      62601    62609       +8     
+ Partials     6389     6384       -5