ref(seer): Call project settings update helper in callsites that don't need to update the full Seer project preference

[srest2021]

Followup to #116352.

Reroute existing callsites that modify seer project settings (_write_preferences_to_sentry_db, configure_seer_for_existing_org, set_default_project_seer_preferences) to the unified update_seer_project_settings helper. This ensures handoff options are cleared/added atomically and skips unnecessary connected repo operations.

Also remove Project row locks from _write_preferences_to_sentry_db and clear_preference_automation_handoff — no longer needed since handoff options are all either cleared or added atomically, and the other update fields may be updated independently of each other.

[srest2021]

We don't sacrifice performance here, because bulk_write_preferences_to_sentry_db used to call _write_preference_project_options which set project options individually even within the same project.

So even though we can't bulk operate across all projects since the updates could differ by project, we're now bulk updating each project which is an improvement.

[srest2021]

We don't want to clear auto_create_pr here:

• We only read a handoff if the other three fields (point, target, integration id) are non-null.
• Legacy seer: if we reenable the agent we want to restore the toggle to its previous state
• Seat-based seer: generally this field should stay in sync with stopping point

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[sentry]

Bug: The clear_preference_automation_handoff function fails to clear the auto_create_pr project option, leaving a stale value that can be incorrectly reused in subsequent handoff configurations.
_{Severity: MEDIUM}

Suggested Fix

Update the clear_preference_automation_handoff function to include SEER_AUTO_CREATE_PR_PROJECT_OPTION in the list of keys passed to ProjectOption.objects.filter(key__in=...).delete(). This will ensure the auto_create_pr setting is properly cleared along with other handoff-related options.

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/seer/autofix/utils.py#L569-L577

Potential issue: The refactored `clear_preference_automation_handoff` function no longer
clears the `sentry:seer_automation_handoff_auto_create_pr` project option. The previous
implementation explicitly deleted this key, but the new implementation, which uses
`ProjectOption.objects.filter(key__in=[...]).delete()`, omits it from the list of keys
to be deleted. This leaves a stale `auto_create_pr=True` value in the database after a
handoff is cleared. If a user later re-enables a handoff without specifying the
`auto_create_pr` value, the stale `True` value will be read, causing the system to
incorrectly enable auto-PR creation when it was not requested for the new configuration.

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