feat: route Codex Chat providers through Stream Check

Codex Chat providers (apiFormat=openai_chat, e.g. DeepSeek, MiniMax, Kimi)
were incorrectly probed via /v1/responses by Stream Check, which the upstream
rejects. Detect chat routing via codex_provider_uses_chat_completions and
issue the probe against /chat/completions with a Chat-shaped body.

Also align URL fallback order with the production CodexAdapter::build_url:
origin-only base URLs (https://api.deepseek.com) must hit /v1/<endpoint>
first; bare /<endpoint> only as a fallback. The previous order made any
non-404 error on the bare path (401/400/405) flag the provider as down even
though /v1/<endpoint> would have succeeded.

- Lift origin-only detection into proxy::providers::is_origin_only_url so
  both build_url and Stream Check share a single source of truth.
- Collapse resolve_codex_stream_urls and resolve_codex_chat_stream_urls into
  a generic resolve_codex_endpoint_urls(base_url, is_full_url, endpoint),
  which also gives the Responses path a symmetric "full endpoint without
  is_full_url flag" fallback for free.
- Restrict reasoning_effort propagation in the Chat branch to OpenAI o-series
  models, mirroring transform_codex_chat's runtime check.

src-tauri/src/proxy/providers/mod.rs

@@ -47,6 +47,7 @@ pub use claude::{
 pub use codex::CodexAdapter;
 pub use codex::{
     apply_codex_chat_upstream_model, codex_provider_upstream_model,
+    codex_provider_uses_chat_completions, is_origin_only_url,
     should_convert_codex_responses_to_chat,
 };
 pub use gemini::GeminiAdapter;