Build the isolated evaluator with the native provider client

Why: The live harness must use the actual client constructor and preserve MiniMax reasoning separation.

Evidence: SELF live-tag compilation passes without provider calls.

Impact: Test harness only; no production route changes.

Refs: fast-slow-u2-u3 Task 7

internal/server/work_checkpoint_eval_test.go

@@ -221,7 +221,7 @@ func evalProviderStream(route map[string]any, dir string) agentllm.StreamFunc {
 	base, _ := route["baseUrl"].(string)
 	key, _ := route["apiKey"].(string)
 	model, _ := route["model"].(string)
-	provider := agentllm.New(base, key, model).Stream
+	provider := (&agentllm.Client{BaseURL: base, APIKey: key, HTTPClient: &http.Client{}, ReasoningSplit: strings.HasPrefix(model, "MiniMax-M")}).Stream
 	type allowance struct {
 		Calls   int
 		Started time.Time