refactor: rename bailian to modelstudio and fix review issues

- Rename provider ID, constants, functions, CLI flags, and types from
  "bailian" to "modelstudio" to match the official English name
  "Alibaba Cloud Model Studio".
- Fix P2 bug: global endpoint variant now always overwrites baseUrl
  instead of silently preserving a stale CN URL.
- Fix P1 bug: add modelstudio entry to PROVIDER_ENV_VARS so
  secret-input-mode=ref no longer throws.
- Move Model Studio imports to top of onboard-auth.config-core.ts.
- Remove unused BAILIAN_BASE_URL export.

Made-with: Cursor

src/cli/program/register.onboard.ts

@@ -160,8 +160,8 @@ export function registerOnboardCommand(program: Command) {
           zaiApiKey: opts.zaiApiKey as string | undefined,
           xiaomiApiKey: opts.xiaomiApiKey as string | undefined,
           qianfanApiKey: opts.qianfanApiKey as string | undefined,
-          bailianApiKeyCn: opts.bailianApiKeyCn as string | undefined,
-          bailianApiKey: opts.bailianApiKey as string | undefined,
+          modelstudioApiKeyCn: opts.modelstudioApiKeyCn as string | undefined,
+          modelstudioApiKey: opts.modelstudioApiKey as string | undefined,
           minimaxApiKey: opts.minimaxApiKey as string | undefined,
           syntheticApiKey: opts.syntheticApiKey as string | undefined,
           veniceApiKey: opts.veniceApiKey as string | undefined,