feat(incidents): Add skip-paging option for self-handled P0/P1 incidents

[rgibert]

Summary

• Adds a "Skip paging on-call responders" checkbox to the /inc new Slack modal, grouped with the private incident toggle in a single Options block
• Checkbox appears only for P0/P1 severity (dynamically via views.update on severity change) and defaults to checked
• When selected, suppresses both PagerDuty paging and on-call channel invites at creation time
• Implemented as a transient flag threaded through serializer context -- no model or migration changes

Agent transcript: https://claudescope.sentry.dev/share/eReflyKHOjlxj3XHUR_WogKuafhwOofLe6_a9YdDSIA

[linear-code]

RELENG-833

[cursor]

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

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[cursor]

Severity change rechecks skip paging

Medium Severity

In handle_severity_action, when severity is P0/P1 the code adds skip_paging to selected_values whenever it is missing. That runs on every severity change, so a user who unchecked “Skip paging on-call responders” to allow paging can have the option forced back on when they switch between high severities, suppressing PagerDuty and on-call invites contrary to their choice.

 

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[sentry]

Bug: The "Skip paging" checkbox is incorrectly re-checked if a user unchecks it, changes to a non-high severity, and then changes back to a high severity.
_{Severity: MEDIUM}

Suggested Fix

Modify the handle_severity_action handler to preserve the user's choice for skip_paging across severity changes. The logic should not automatically re-add the skip_paging default if the user has previously interacted with the incident creation form. This could involve checking the prior state more carefully before injecting default values.

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/firetower/slack_app/handlers/new_incident.py#L276-L277

Potential issue: When a user creates a high-severity incident (P0/P1), explicitly
unchecks the "Skip paging on-call responders" option, changes the severity to a non-high
level (e.g., P2), and then changes it back to a high severity, the "Skip paging" option
is incorrectly re-checked. This happens because the logic at `handle_severity_action`
re-injects the `skip_paging` default when it's not found in the view's state. The state
is cleared of this option when switching to a non-high severity, causing the system to
forget the user's explicit choice to uncheck the box, leading to unwanted PagerDuty
pages.

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