Drop duo_agent_platform_request_count from namespace_settings and ai_settings

Summary

• Drop duo_agent_platform_request_count column from namespace_settings and ai_settings tables via post-deploy migrations
• The ignore_column declarations in NamespaceSetting and Ai::Setting models are still present and will be cleaned up in a follow-up MR
• Both columns were ignored past their remove_with dates (18.8 and 18.9, current milestone is 18.10)

Details

The duo_agent_platform_request_count column was originally added in 18.4 but is no longer used. It was marked as ignored via ignore_column with the following timelines:

• ai_settings: remove_with: '18.8', remove_after: '2025-12-13'
• namespace_settings: remove_with: '18.9', remove_after: '2026-01-16'

Both dates have passed, so this MR drops the columns via post-deploy migrations. The ignore_column declarations will be removed in a follow-up MR.

Closes #579366 (closed)

Test plan

• Migrations run successfully against development database
• structure.sql updated — no references to duo_agent_platform_request_count
• No remaining references in models, specs, or config files
• CI pipeline passes