[codex] Dedupe fallback model metadata warnings (#21090)

Fixes #21070.

This is a small cleanup around model metadata handling for
gateway/provider model names. It follows the report and proposed
direction from @dkbush by keeping the fallback metadata warning useful
without repeating it every turn, and by tightening the existing
provider-prefix lookup path.

- Track fallback metadata warning slugs in session state so each
unresolved model warns once per session.
- Keep warning emission outside the session-state lock and preserve the
existing warning text.
- Allow one-segment provider prefixes with hyphenated provider IDs,
while preserving the multi-segment rejection behavior.
- Add focused coverage for warning dedupe and hyphenated provider-prefix
metadata matching.

Testing:

- Ran `just fmt`.
- Ran `git diff --check`.
- Added tests for the new warning dedupe and provider-prefix lookup
behavior.

codex-rs/models-manager/src/manager.rs

@@ -421,15 +421,16 @@ fn find_model_by_longest_prefix(model: &str, candidates: &[ModelInfo]) -> Option
 fn find_model_by_namespaced_suffix(model: &str, candidates: &[ModelInfo]) -> Option<ModelInfo> {
     // Retry metadata lookup for a single namespaced slug like `namespace/model-name`.
     //
-    // This only strips one leading namespace segment and only when the namespace is ASCII
-    // alphanumeric/underscore (`\w+`) to avoid broadly matching arbitrary aliases.
+    // This only strips one leading namespace segment and only when the namespace looks
+    // like a simple provider id to avoid broadly matching arbitrary aliases.
     let (namespace, suffix) = model.split_once('/')?;
     if suffix.contains('/') {
         return None;
     }
-    if !namespace
-        .chars()
-        .all(|c| c.is_ascii_alphanumeric() || c == '_')
+    if namespace.is_empty()
+        || !namespace
+            .chars()
+            .all(|c| c.is_ascii_alphanumeric() || c == '_' || c == '-')
     {
         return None;
     }