override verbosity for gpt-5-codex (#6007)

we are seeing [reports](https://github.com/openai/codex/issues/6004) of
users having verbosity in their config.toml and facing issues.
gpt-5-codex doesn't accept other values rather than medium for
verbosity.

codex-rs/core/src/client.rs

@@ -216,10 +216,12 @@ impl ModelClient {
         let verbosity = if self.config.model_family.support_verbosity {
             self.config.model_verbosity
         } else {
-            warn!(
-                "model_verbosity is set but ignored as the model does not support verbosity: {}",
-                self.config.model_family.family
-            );
+            if self.config.model_verbosity.is_some() {
+                warn!(
+                    "model_verbosity is set but ignored as the model does not support verbosity: {}",
+                    self.config.model_family.family
+                );
+            }
             None
         };