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/ProviderForm.tsx

@@ -447,6 +447,7 @@ export function ProviderForm({
     handleCommonConfigSnippetChange: handleCodexCommonConfigSnippetChange,
     isExtracting: isCodexExtracting,
     handleExtract: handleCodexExtract,
+    clearCommonConfigError: clearCodexCommonConfigError,
   } = useCodexCommonConfig({
     codexConfig,
     onConfigChange: handleCodexConfigChange,
@@ -530,6 +531,7 @@ export function ProviderForm({
     handleCommonConfigSnippetChange: handleGeminiCommonConfigSnippetChange,
     isExtracting: isGeminiExtracting,
     handleExtract: handleGeminiExtract,
+    clearCommonConfigError: clearGeminiCommonConfigError,
   } = useGeminiCommonConfig({
     envValue: geminiEnv,
     onEnvChange: handleGeminiEnvChange,
@@ -1467,6 +1469,7 @@ export function ProviderForm({
               onCommonConfigToggle={handleCodexCommonConfigToggle}
               commonConfigSnippet={codexCommonConfigSnippet}
               onCommonConfigSnippetChange={handleCodexCommonConfigSnippetChange}
+              onCommonConfigErrorClear={clearCodexCommonConfigError}
               commonConfigError={codexCommonConfigError}
               authError={codexAuthError}
               configError={codexConfigError}
@@ -1488,6 +1491,7 @@ export function ProviderForm({
               onCommonConfigSnippetChange={
                 handleGeminiCommonConfigSnippetChange
               }
+              onCommonConfigErrorClear={clearGeminiCommonConfigError}
               commonConfigError={geminiCommonConfigError}
               envError={envError}
               configError={geminiConfigError}