Switch Ai::DuoWorkflows::Checkpoint model to use the partitioned table

What does this MR do and why?

Switch Ai::DuoWorkflows::Checkpoint model to use the partitioned table

duo_agent_platform_checkpoints is the daily partitioned table replacing duo_workflows_checkpoints table.

Depends on : Add `p_duo_workflows_checkpoints` daily partiti... (!193398 - merged)

1. ✅ "Add duo_agent_platform_checkpoints table with partitioning". The table will stay idle until step two.

2. ➡️ Switch the rails model Ai::DuoWorkflows::Checkpoint to the new table

• a. Disable the main feature flag DW/AC (Estimated avg: 6 hours stop during private beta)
• b. ➡️ (the current MR) Deploy the "MR to switch the checkpoint" rails model to use the new table, including partitioning changes. This ensures rails restarts and caches database schema from the new table and prevents canary issues
• c. Re-enable the main feature flag DW/AC

References

• Previous MR Add `p_duo_workflows_checkpoints` daily partiti... (!193398 - merged)

How to set up and validate locally

• Related to Partition the `duo_workflows_checkpoints` table... (#470469 - closed)

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