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

src/components/providers/forms/GeminiConfigEditor.tsx

@@ -1,4 +1,4 @@
-import React, { useState, useEffect } from "react";
+import React, { useState } from "react";
 import { GeminiEnvSection, GeminiConfigSection } from "./GeminiConfigSections";
 import { GeminiCommonConfigModal } from "./GeminiCommonConfigModal";
 
@@ -11,7 +11,8 @@ interface GeminiConfigEditorProps {
   useCommonConfig: boolean;
   onCommonConfigToggle: (checked: boolean) => void;
   commonConfigSnippet: string;
-  onCommonConfigSnippetChange: (value: string) => void;
+  onCommonConfigSnippetChange: (value: string) => boolean;
+  onCommonConfigErrorClear: () => void;
   commonConfigError: string;
   envError: string;
   configError: string;
@@ -29,6 +30,7 @@ const GeminiConfigEditor: React.FC<GeminiConfigEditorProps> = ({
   onCommonConfigToggle,
   commonConfigSnippet,
   onCommonConfigSnippetChange,
+  onCommonConfigErrorClear,
   commonConfigError,
   envError,
   configError,
@@ -37,12 +39,10 @@ const GeminiConfigEditor: React.FC<GeminiConfigEditorProps> = ({
 }) => {
   const [isCommonConfigModalOpen, setIsCommonConfigModalOpen] = useState(false);
 
-  // Auto-open common config modal if there's an error
-  useEffect(() => {
-    if (commonConfigError && !isCommonConfigModalOpen) {
-      setIsCommonConfigModalOpen(true);
-    }
-  }, [commonConfigError, isCommonConfigModalOpen]);
+  const handleCloseCommonConfigModal = () => {
+    onCommonConfigErrorClear();
+    setIsCommonConfigModalOpen(false);
+  };
 
   return (
     <div className="space-y-6">
@@ -68,9 +68,9 @@ const GeminiConfigEditor: React.FC<GeminiConfigEditorProps> = ({
       {/* Common Config Modal */}
       <GeminiCommonConfigModal
         isOpen={isCommonConfigModalOpen}
-        onClose={() => setIsCommonConfigModalOpen(false)}
+        onClose={handleCloseCommonConfigModal}
         value={commonConfigSnippet}
-        onChange={onCommonConfigSnippetChange}
+        onSave={onCommonConfigSnippetChange}
         error={commonConfigError}
         onExtract={onExtract}
         isExtracting={isExtracting}