chore(group_history): Safe removal of prev_history

[armenzg]

This is part of the required steps to remove a column from Sentry:
https://develop.sentry.dev/backend/application-domains/database-migrations/#deleting-columns.

This is a follow-up to #102474.

[codecov]

Codecov Report

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

Additional details and impacted files

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- Misses      74224     74236      +12     
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[armenzg]

This causes integrity errors like this: https://sentry.sentry.io/issues/6994851757/

[armenzg]

This simply marks the column as not useable.

[sentry]

Bug: Removing prev_history from GroupHistory model before its SafeRemoveField migration runs causes a FieldDoesNotExist error during migration execution.
_{Severity: CRITICAL | Confidence: 1.00}

🔍 Detailed Analysis

The SafeRemoveField.state_forwards() method, used in the migration, expects the prev_history field to exist in the GroupHistory model definition when the migration runs. Removing prev_history from src/sentry/models/grouphistory.py at lines 215-217 in the same change as the migration causes state.apps.get_model(...)._meta.get_field('prev_history') to raise a FieldDoesNotExist error, leading to a migration crash.

💡 Suggested Fix

Retain the prev_history field definition in src/sentry/models/grouphistory.py in this PR. Remove the field definition in a separate, subsequent PR after the migration has successfully deployed and executed in production.

🤖 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/models/grouphistory.py#L215-L217

Potential issue: The `SafeRemoveField.state_forwards()` method, used in the migration,
expects the `prev_history` field to exist in the `GroupHistory` model definition when
the migration runs. Removing `prev_history` from `src/sentry/models/grouphistory.py` at
lines 215-217 in the same change as the migration causes
`state.apps.get_model(...)._meta.get_field('prev_history')` to raise a
`FieldDoesNotExist` error, leading to a migration crash.

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

[github-actions]

This PR has a migration; here is the generated SQL for src/sentry/migrations/1002_group_history_prev_history_remove_db_constraint_and_safe_removal.py

for 1002_group_history_prev_history_remove_db_constraint_and_safe_removal in sentry

--
-- Alter field prev_history on grouphistory
--
SET CONSTRAINTS "sentry_grouphistory_prev_history_id_905ec402_fk_sentry_gr" IMMEDIATE; ALTER TABLE "sentry_grouphistory" DROP CONSTRAINT "sentry_grouphistory_prev_history_id_905ec402_fk_sentry_gr";
--
-- Moved grouphistory.prev_history field to pending deletion state
--
-- (no-op)

[armenzg]

I will open a new PR. What a mess.