feat(proxy): add GET /v1/models endpoint for Codex CLI reachability check (#3818)

* feat(proxy): add GET /v1/models endpoint for Codex CLI reachability check

Codex CLI probes GET /v1/models at startup. Without this endpoint the proxy
returns 404, causing Codex to fail before any request reaches the upstream
LLM.

Return an OpenAI-compatible model list derived from the cc-switch–managed
model catalog file.

Fixes #3812

* fix(proxy): return Codex catalog schema from /v1/models

Codex deserializes the response as a catalog with a top-level `models`
field, not the OpenAI `{"object":"list","data":[...]}` envelope.
Return the catalog file content directly so the format matches what
Codex expects.

Co-authored-by: Codex review bot

* fix(proxy): guard /v1/models against serving stale catalog

Only return the model catalog when config.toml still references it via
`model_catalog_json`.  After switching to a provider without a custom
catalog, the old file lingers on disk — serving it unconditionally
would advertise the previous provider's models to Codex.

Co-authored-by: Codex review bot

* fix(proxy): match relative model_catalog_json in stale-guard

cc-switch writes `model_catalog_json = "cc-switch-model-catalog.json"`
(relative) via set_codex_model_catalog_json_field.  Match on the
filename constant rather than the absolute path so the guard works
with both relative and absolute paths.

Co-authored-by: Codex review bot

* fix(proxy): parse model_catalog_json field instead of substring match

Replace raw config_text.contains() with proper TOML field parsing so
commented-out lines and stray mentions of the filename in other fields
don't defeat the stale guard.  Also switch from contains() to exact
filename match (Path::new(val).file_name() == Some(...)) to stay
consistent with resolve_cc_switch_catalog_path in codex_config.rs.

Add log::debug! when the guard blocks serving so the operator can
distinguish "no models configured" from "guard blocked stale catalog".

* refactor(proxy): reuse resolve_cc_switch_catalog_path in handle_models

Replace the inline config.toml parsing and filename match in
handle_models with the existing resolve_cc_switch_catalog_path helper
(now pub(crate)). This removes the duplicated stale-guard logic, keeps
a single source of truth for catalog-path ownership, and makes the
handler honor absolute model_catalog_json paths the same way Codex
live-setting import does.

---------

Co-authored-by: Jason <farion1231@gmail.com>

src-tauri/src/codex_config.rs

@@ -766,7 +766,10 @@ pub fn read_codex_model_catalog_simplified_from_live() -> Result<Option<Value>,
 /// Given `config.toml` text, resolve the on-disk path of the cc-switch–owned
 /// catalog file (returns `None` if `model_catalog_json` is absent or points at
 /// a file we don't own). Relative paths fall back to `generated_path`.
-fn resolve_cc_switch_catalog_path(config_text: &str, generated_path: &Path) -> Option<PathBuf> {
+pub(crate) fn resolve_cc_switch_catalog_path(
+    config_text: &str,
+    generated_path: &Path,
+) -> Option<PathBuf> {
     if config_text.trim().is_empty() {
         return None;
     }