fix(api): unify recipient cap errors

[jiashuoz]

Summary

• route all outbound recipient-count violations through the documented 400 too_many_recipients envelope
• count approval overrides after merging with the stored held draft
• keep the combined 50-recipient cap discoverable in OpenAPI descriptions and regenerate both SDK bases

Test plan

• go test ./internal/httpapi -count=1
• go test ./internal/agent -run TestApprovePendingCore -count=1
• make generate-check
• npm run build --workspace @e2a/sdk