Honor personality = "none" in model instructions (#32277)

## Why

Model catalog instructions can include a baked-in `# Personality` section. An
explicit `none` setting should omit that section instead of sending it as part
of the model's base instructions.

## What changed

- Pass the configured personality into the models manager.
- When personality support is enabled and the setting is explicitly `none`,
  remove the `# Personality` section through the next level-one heading from
  catalog base instructions and instruction templates.
- Preserve explicit `base_instructions` overrides and avoid warning when no
  personality was requested.

## Testing

Added unit and integration coverage for section removal, heading boundaries,
CRLF input, preserved configurations, and explicit base instructions.

GitOrigin-RevId: 452c88d3ac6001c2ac7d4fef269cd75dc239fa61

codex-rs/protocol/src/openai_models.rs

@@ -465,14 +465,17 @@ impl ModelInfo {
                 .get_personality_message(personality)
                 .unwrap_or_default();
             template.replace(PERSONALITY_PLACEHOLDER, personality_message.as_str())
-        } else if let Some(personality) = personality {
-            warn!(
-                model = %self.slug,
-                %personality,
-                "Model personality requested but model_messages is missing, falling back to base instructions."
-            );
-            self.base_instructions.clone()
         } else {
+            match personality {
+                Some(personality @ (Personality::Friendly | Personality::Pragmatic)) => {
+                    warn!(
+                        model = %self.slug,
+                        %personality,
+                        "Model personality requested but model_messages is missing, falling back to base instructions."
+                    );
+                }
+                Some(Personality::None) | None => {}
+            }
             self.base_instructions.clone()
         }
     }