fix(go-agent): preserve realtime stream deltas (#17791)

- Start the Agent model-stream collector before ReAct execution.
- Preserve all streamed reasoning/content deltas and drain the collector
on errors.
- Add coverage for delayed thinking streams and tool-call execution.

internal/agent/tool/code_exec_test.go

@@ -91,6 +91,68 @@ func TestCodeExec_Info(t *testing.T) {
 	if !strings.Contains(info.Desc, "Python") {
 		t.Errorf("Desc = %q, want to mention Python", info.Desc)
 	}
+
+	params, err := info.ParamsOneOf.ToJSONSchema()
+	if err != nil {
+		t.Fatalf("Info schema: %v", err)
+	}
+	encoded, err := json.Marshal(params)
+	if err != nil {
+		t.Fatalf("marshal Info schema: %v", err)
+	}
+	var schema map[string]any
+	if err := json.Unmarshal(encoded, &schema); err != nil {
+		t.Fatalf("decode Info schema: %v", err)
+	}
+	properties, ok := schema["properties"].(map[string]any)
+	if !ok {
+		t.Fatalf("Info schema properties = %#v, want object", schema["properties"])
+	}
+	for _, name := range []string{"lang", "script"} {
+		if _, ok := properties[name]; !ok {
+			t.Errorf("Info schema missing %q", name)
+		}
+	}
+	for _, name := range []string{"language", "code", "arguments", "outputs"} {
+		if _, ok := properties[name]; ok {
+			t.Errorf("Info schema unexpectedly exposes node field %q", name)
+		}
+	}
+	required, ok := schema["required"].([]any)
+	if !ok {
+		t.Fatalf("Info schema required = %#v, want array", schema["required"])
+	}
+	requiredFields := make(map[string]bool, len(required))
+	for _, field := range required {
+		if name, ok := field.(string); ok {
+			requiredFields[name] = true
+		}
+	}
+	if !requiredFields["lang"] || !requiredFields["script"] {
+		t.Errorf("Info schema required = %#v, want lang and script", required)
+	}
+	langProp, ok := properties["lang"].(map[string]any)
+	if !ok {
+		t.Fatalf("lang property = %#v, want object", properties["lang"])
+	}
+	if typ, _ := langProp["type"].(string); typ != "string" {
+		t.Errorf("lang.type = %q, want string", typ)
+	}
+	enum, ok := langProp["enum"].([]any)
+	if !ok {
+		t.Fatalf("lang.enum = %#v, want array", langProp["enum"])
+	}
+	gotEnum := make([]string, len(enum))
+	for i, e := range enum {
+		s, ok := e.(string)
+		if !ok {
+			t.Fatalf("lang.enum[%d] = %#v, want string", i, e)
+		}
+		gotEnum[i] = s
+	}
+	if len(gotEnum) != 2 || gotEnum[0] != "python" || gotEnum[1] != "javascript" {
+		t.Errorf("lang.enum = %v, want [python javascript]", gotEnum)
+	}
 }
 
 // TestCodeExec_ResultExtractsArtifacts pins the artifact