feat(usage): add support for tracking request source in usage records

- Introduced `Source` field to usage-related structs for better origin tracking.
- Updated `newUsageReporter` to resolve and populate the `Source` attribute.
- Implemented `resolveUsageSource` to determine source from auth metadata or API key.

sdk/cliproxy/usage/manager.go

@@ -14,6 +14,7 @@ type Record struct {
 	Model       string
 	APIKey      string
 	AuthID      string
+	Source      string
 	RequestedAt time.Time
 	Detail      Detail
 }