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/core/src/config/mod.rs

@@ -1473,6 +1473,7 @@ impl Config {
             tool_output_token_limit: self.tool_output_token_limit,
             base_instructions: self.base_instructions.clone(),
             personality_enabled: self.features.enabled(Feature::Personality),
+            personality: self.personality,
             model_catalog: self.model_catalog.clone(),
         }
     }