fix: resolve cherry-pick compilation and test issues

- Add int64(0) param to SelectAccountWithLoadAwareness callers (signature change from channel scheduling refactor)
- Add UsageMapHook type and struct field to StreamingProcessor
- Revert Claude Max cache billing code to upstream/main (not part of channel feature)
- Revert credits overages logic to upstream/main (non-channel change)
- Remove Instructions field reference (non-channel OpenAI feature)
- Restore sora_client_handler_test.go from upstream + add channel service nil params

backend/internal/pkg/apicompat/chatcompletions_to_responses.go

@@ -27,13 +27,12 @@ func ChatCompletionsToResponses(req *ChatCompletionsRequest) (*ResponsesRequest,
 	}
 
 	out := &ResponsesRequest{
-		Model:        req.Model,
-		Instructions: req.Instructions,
-		Input:        inputJSON,
-		Temperature:  req.Temperature,
-		TopP:         req.TopP,
-		Stream:       true, // upstream always streams
-		Include:      []string{"reasoning.encrypted_content"},
+		Model:       req.Model,
+		Input:       inputJSON,
+		Temperature: req.Temperature,
+		TopP:        req.TopP,
+		Stream:      true, // upstream always streams
+		Include:     []string{"reasoning.encrypted_content"},
 		ServiceTier:  req.ServiceTier,
 	}