fix(form): add type guard for Gemini env values

Address code audit finding #10:

Replace unsafe type assertion with proper type guard when extracting
custom env config. Convert all values to strings explicitly using
String() to prevent runtime type errors when env contains non-string
values (numbers, booleans, etc).

Before: envObj = customConfig as Record<string, string>
After: Iterate and convert each value with String(value)

src/components/providers/forms/ProviderForm.tsx

@@ -944,7 +944,12 @@ export function ProviderForm({
           const commonEnvObj = envStringToObj(geminiCommonConfigSnippet.trim());
           if (isPlainObject(envObj) && isPlainObject(commonEnvObj)) {
             const { customConfig } = extractDifference(envObj, commonEnvObj);
-            envObj = customConfig as Record<string, string>;
+            // Convert to string record with type guard to avoid type assertion issues
+            const stringEnvObj: Record<string, string> = {};
+            for (const [key, value] of Object.entries(customConfig)) {
+              stringEnvObj[key] = String(value);
+            }
+            envObj = stringEnvObj;
           }
         }
         const combined = {