feat: add Mark All as Read feature for inbox notifications

[kylecarbs]

Summary

• Implements feature request from issue #15264 to add a "Mark all as read" feature for notifications
• Adds a new API endpoint and SDK method to mark all notifications as read for a user

Details

This PR adds:

1. A new SQL query to mark all unread notifications as read for a user
2. A new REST API endpoint at /api/v2/notifications/inbox/mark-all-read
3. A new SDK method MarkAllInboxNotificationsRead
4. A comprehensive test to verify the functionality

Fixes #15264

Test plan

• Manual testing: Open notifications panel and use mark all as read button
• Automated tests: Added test in TestInboxNotifications_MarkAllRead which verifies that:
  • All notifications are properly marked as read
  • Unread count is properly updated to 0

🤖 Generated with Claude Code

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