refactor(common-config): consolidate hooks and migrate Gemini to ENV format

- Delete redundant wrapper hooks (useCommonConfigSnippet, useGeminiCommonConfig)
- Change Gemini common config from JSON to ENV format (.env style)
- Add backend validation with forbidden keys filtering (GEMINI_API_KEY, GOOGLE_GEMINI_BASE_URL)
- Fix localStorage migration to skip empty parsed snippets
- Add error handling for silent JSON parse failures
- Clean up debug logs and unused types

src/components/providers/forms/hooks/useCodexCommonConfig.ts

@@ -10,12 +10,6 @@ import { codexAdapter } from "@/hooks/commonConfigAdapters";
 import { extractTomlDifference } from "@/utils/tomlConfigMerge";
 import type { ProviderMeta } from "@/types";
 
-/** TOML 校验错误码 */
-export type TomlValidationErrorCode =
-  | "TOML_SYNTAX_ERROR"
-  | "TOML_PARSE_FAILED"
-  | "";
-
 interface UseCodexCommonConfigProps {
   /**
    * 当前 Codex 配置（可能是纯 TOML 或 JSON wrapper 格式）