chore(ACI): Remove temporary fallthrough type logic

[ceorourke]

Follow up to #103764 - once the migration is complete we can remove the temporary logic to support the incorrect case.

[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   #103996      +/-   ##
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+ Coverage   74.95%    80.62%   +5.67%     
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  Files        9311      9313       +2     
  Lines      397391    397415      +24     
  Branches    25358     25358              
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+ Hits       297860    320425   +22565     
+ Misses      99074     76533   -22541     
  Partials      457       457              

[sentry]

Bug: Removing fallthroughType backward compatibility before post-deployment migration causes TypeError.
_{Severity: CRITICAL | Confidence: High}

🔍 Detailed Analysis

The code removes backward compatibility logic for the fallthroughType field before its corresponding post-deployment migration has run. This causes a TypeError crash when EmailDataBlob(**action.data) is called with existing Action records that still contain fallthroughType (camelCase) in their action.data, as EmailDataBlob only accepts fallthrough_type (snake_case). This affects email notifications to issue owners immediately after deployment and before the migration 0104_action_data_fallthrough_type.py completes.

💡 Suggested Fix

Reinstate the backward compatibility logic to convert fallthroughType to fallthrough_type before passing action.data to EmailDataBlob, or ensure the migration runs before the code deployment.

🤖 Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent.
Verify if this is a real issue. If it is, propose a fix; if not, explain why it's not
valid.

Location:
src/sentry/notifications/notification_action/issue_alert_registry/handlers/email_issue_alert_handler.py#L54

Potential issue: The code removes backward compatibility logic for the `fallthroughType`
field before its corresponding post-deployment migration has run. This causes a
`TypeError` crash when `EmailDataBlob(**action.data)` is called with existing `Action`
records that still contain `fallthroughType` (camelCase) in their `action.data`, as
`EmailDataBlob` only accepts `fallthrough_type` (snake_case). This affects email
notifications to issue owners immediately after deployment and before the migration
`0104_action_data_fallthrough_type.py` completes.

_{Did we get this right? 👍 / 👎 to inform future reviews.
Reference ID: 3588791}