Update pricing multipliers for gpt-5.1-codex and gpt-5.2-codex models

Problem

For ai_gateway_proxy_use events, we're receiving model values gpt-5.1-codex and gpt-5.2-codex for which we don't have pricing multipliers configured. This is causing:

• Monitoring channel noise from missing pricing multiplier errors
• Inability to properly bill ai_gateway_proxy_use events

Current Solution

As a temporary measure, we've added a pricing multiplier of 0 for these models to suppress the errors. This allows the system to continue without errors, but prevents accurate billing if these are to be billed. https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/14673+.

Required Actions

1. Obtain the actual pricing multiplier values for:
   • gpt-5.1-codex
   • gpt-5.2-codex
2. Remove the temporary 0 multiplier and update the pricing configuration with the correct multiplier values
3. Remove quarantine for the validates multiplier value is positive when present spec in spec/configs/pricing_multipliers_config_spec.rb

Notes

• ai_gateway_proxy_use events are currently not billable
• This issue should be resolved before making these events billable in production