fix: normalize image billing inputs - #7

Image billing expressions currently see raw request fields, so invalid/defaulted n and provider-specific sizes can produce the wrong quota.

This change:

• feeds validated/defaulted image counts into billing expressions
• normalizes image sizes to the same 1K/2K/4K buckets used by the upstream
• preserves custom request fields in live and channel-test paths
• rejects non-object image billing bodies without panicking

Verification:

• go test ./dto ./relay/helper ./controller ./model ./relay/channel/minimax
• git diff --check
• Independent review: PASS (gpt-5.6-sol, medium)

Agent: gpt-5.6-sol via T3 Code
Review: gpt-5.6-sol (medium) via T3 Code