ref(issue summary): Refactor issue summary to make logic reusable

[roaga]

Making issue summary logic reusable in preparation for triggering from alerts. Should not affect functionality.

[codecov]

Codecov Report

Attention: Patch coverage is 93.04348% with 24 lines in your changes missing coverage. Please review.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/sentry/seer/issue_summary.py	79.83%	24 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #87822   +/-   ##
=======================================
  Coverage   87.76%   87.76%           
=======================================
  Files        9907     9909    +2     
  Lines      562666   562713   +47     
  Branches    22128    22128           
=======================================
+ Hits       493837   493893   +56     
+ Misses      68398    68389    -9     
  Partials      431      431           

[sentry-io]

Suspect Issues

This pull request was deployed and Sentry observed the following issues:

• ‼️ AttributeError: 'NoneType' object has no attribute 'get' /api/0/organizations/{organization_id_or_slug}/... View Issue
• ‼️ HTTPError: 500 Server Error: Internal Server Error for url: http://seer-web-autofix/v1/automation/summarize/issue /api/0/organizations/{organization_id_or_slug}/... View Issue

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