fix: prevent common config modal infinite reopen loop and add draft editing

The auto-open useEffect in CodexConfigEditor and GeminiConfigEditor
created an inescapable loop: commonConfigError triggered modal open,
closing the modal didn't clear the error, so the effect immediately
reopened it — locking the entire UI.

- Remove auto-open useEffect from both Codex and Gemini config editors
- Convert common config modals to draft editing (edit locally, validate
  before save) instead of persisting on every keystroke
- Add TOML/JSON pre-validation via parseCommonConfigSnippet before any
  merge operation to prevent invalid content from being persisted
- Expose clearCommonConfigError so editors can clear stale errors on
  modal close

tests/hooks/useCommonConfigSave.test.tsx

@@ -0,0 +1,79 @@
+import { act, renderHook, waitFor } from "@testing-library/react";
+import { beforeEach, describe, expect, it, vi } from "vitest";
+import { useCodexCommonConfig } from "@/components/providers/forms/hooks/useCodexCommonConfig";
+import { useGeminiCommonConfig } from "@/components/providers/forms/hooks/useGeminiCommonConfig";
+
+const getCommonConfigSnippetMock = vi.fn();
+const setCommonConfigSnippetMock = vi.fn();
+const extractCommonConfigSnippetMock = vi.fn();
+
+vi.mock("@/lib/api", () => ({
+  configApi: {
+    getCommonConfigSnippet: (...args: unknown[]) =>
+      getCommonConfigSnippetMock(...args),
+    setCommonConfigSnippet: (...args: unknown[]) =>
+      setCommonConfigSnippetMock(...args),
+    extractCommonConfigSnippet: (...args: unknown[]) =>
+      extractCommonConfigSnippetMock(...args),
+  },
+}));
+
+describe("common config snippet saving", () => {
+  beforeEach(() => {
+    getCommonConfigSnippetMock.mockResolvedValue("");
+    setCommonConfigSnippetMock.mockResolvedValue(undefined);
+    extractCommonConfigSnippetMock.mockResolvedValue("");
+  });
+
+  it("does not persist an invalid Codex common config snippet", async () => {
+    const onConfigChange = vi.fn();
+    const { result } = renderHook(() =>
+      useCodexCommonConfig({
+        codexConfig: "model = \"gpt-5\"",
+        onConfigChange,
+      }),
+    );
+
+    await waitFor(() => expect(result.current.isLoading).toBe(false));
+
+    let saved = false;
+    act(() => {
+      saved = result.current.handleCommonConfigSnippetChange(
+        "base_url = https://bad.example/v1",
+      );
+    });
+
+    expect(saved).toBe(false);
+    expect(setCommonConfigSnippetMock).not.toHaveBeenCalled();
+    expect(onConfigChange).not.toHaveBeenCalled();
+    expect(result.current.commonConfigError).toContain("invalid value");
+  });
+
+  it("does not persist an invalid Gemini common config snippet", async () => {
+    const onEnvChange = vi.fn();
+    const { result } = renderHook(() =>
+      useGeminiCommonConfig({
+        envValue: "",
+        onEnvChange,
+        envStringToObj: () => ({}),
+        envObjToString: () => "",
+      }),
+    );
+
+    await waitFor(() => expect(result.current.isLoading).toBe(false));
+
+    let saved = false;
+    act(() => {
+      saved = result.current.handleCommonConfigSnippetChange(
+        JSON.stringify({ GEMINI_MODEL: 123 }),
+      );
+    });
+
+    expect(saved).toBe(false);
+    expect(setCommonConfigSnippetMock).not.toHaveBeenCalled();
+    expect(onEnvChange).not.toHaveBeenCalled();
+    expect(result.current.commonConfigError).toBe(
+      "geminiConfig.commonConfigInvalidValues",
+    );
+  });
+});