fix(codex): avoid creating an empty default provider

[Charlie-1909]

Summary

• avoid writing model_provider or creating [model_providers.codeg] when Codex has neither a configured provider nor a non-empty API base URL
• preserve an existing provider and honor an explicitly requested provider when a non-empty URL is supplied
• keep global responses_websockets_v2 handling independent from provider creation
• apply the same provider-write guard in the TypeScript settings path and the Rust persistence/cascade paths
• preserve the explicit authentication defaults introduced for Bug: Codex requires_openai_auth = false 会被 Codeg 无条件重置为 true #406

Fixes #520

Local verification

• targeted Vitest: 109/109 passed
• targeted ESLint: passed
• production build: passed
• full Vitest run: 4,463/4,466 passed locally; the three failures are in unchanged files:
  • one 5-second timeout passed 11/11 when rerun in isolation
  • two locale-sensitive assertions expect 51,777, while the French Windows locale renders 51?777
• Rust is unavailable in the local environment; this draft PR is opened specifically to run the repository's Rust matrix on Linux, macOS, and Windows

Scope

Only these files differ from main:

• src/components/settings/acp-agent-settings.tsx
• src/components/settings/acp-agent-settings.test.tsx
• src-tauri/src/commands/acp.rs