Add request preparation baseline and boundary tests

internal/llm/openai_compatible_client_test.go

@@ -1113,6 +1113,13 @@ func TestOpenAICompatibleClientRejectsInvalidExtraParamsBeforeProviderCall(t *te
 	}{
 		{name: "empty key", extraParams: map[string]any{"": "empty"}, want: "key must not be empty"},
 		{name: "unserializable value", extraParams: map[string]any{"bad": math.Inf(1)}, want: "JSON-serializable"},
+		{
+			name: "nested unserializable value",
+			extraParams: map[string]any{
+				"nested": map[string]any{"values": []any{"valid", math.NaN()}},
+			},
+			want: "extra_params.nested",
+		},
 	}
 	for _, key := range []string{
 		"model",
@@ -1161,6 +1168,56 @@ func TestOpenAICompatibleClientRejectsInvalidExtraParamsBeforeProviderCall(t *te
 	}
 }
 
+func TestOpenAICompatibleClientClassifiesSchemaEncodingFailures(t *testing.T) {
+	tests := []struct {
+		name        string
+		extraParams map[string]any
+		want        error
+		unwanted    error
+	}{
+		{
+			name:        "serializable extra keeps full-request failure",
+			extraParams: map[string]any{"client_option": "accepted"},
+			want:        ErrRequestFailed,
+			unwanted:    ErrInvalidRequest,
+		},
+		{
+			name:        "invalid extra keeps invalid-request failure",
+			extraParams: map[string]any{"nested": map[string]any{"bad": math.Inf(1)}},
+			want:        ErrInvalidRequest,
+			unwanted:    ErrRequestFailed,
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			provider := newRecordingProvider(t)
+			client := newProviderClient(t, provider, OpenAICompatibleConfig{})
+			request := ordinaryGenerateRequest()
+			request.Target.ExtraParams = tc.extraParams
+			request.StructuredOutput = &domain.StructuredOutputSpec{
+				Type: domain.StructuredOutputJSONSchema,
+				JSONSchema: &domain.StructuredOutputJSONSpec{
+					Name:   "invalid-schema",
+					Strict: true,
+					Schema: json.RawMessage("{"),
+				},
+			}
+
+			_, err := client.Generate(context.Background(), request)
+			if err == nil || !errors.Is(err, tc.want) {
+				t.Fatalf("error = %v, want identity %v", err, tc.want)
+			}
+			if errors.Is(err, tc.unwanted) {
+				t.Fatalf("error = %v, unexpectedly has identity %v", err, tc.unwanted)
+			}
+			if calls := provider.requestCount(); calls != 0 {
+				t.Fatalf("provider calls = %d, want 0", calls)
+			}
+		})
+	}
+}
+
 func checkConfiguredModelFallback(t *testing.T) {
 	provider := newRecordingProvider(t)
 	client := newProviderClient(t, provider, OpenAICompatibleConfig{Model: "default-model"})