Fix inherited availability metadata for Bedrock models (#30897)

## Why

Amazon Bedrock's static catalog derives its GPT model definitions from
bundled OpenAI model metadata. The GPT-5.6 variants introduced in #30285
clone GPT-5.5, which carries an `availability_nux`; because app-server
forwards that metadata through `model/list`, clients can show GPT-5.5
launch copy for a GPT-5.6 Bedrock model.

`upgrade` is also model-catalog availability metadata and should not be
inherited by provider-specific Bedrock models.

## What changed

- Clear `availability_nux` and `upgrade` when constructing static
Bedrock GPT models.
- Add a regression test asserting that every static Bedrock model omits
both fields.

## Testing

- `just test -p codex-model-provider`

codex-rs/model-provider/src/amazon_bedrock/catalog.rs

@@ -61,6 +61,8 @@ fn gpt_5_bedrock_model(openai_slug: &str, bedrock_slug: &str, priority: i32) ->
     model.priority = priority;
     model.context_window = Some(GPT_5_BEDROCK_CONTEXT_WINDOW);
     model.max_context_window = Some(GPT_5_BEDROCK_CONTEXT_WINDOW);
+    model.availability_nux = None;
+    model.upgrade = None;
     model
 }
 
@@ -127,6 +129,15 @@ mod tests {
         }
     }
 
+    #[test]
+    fn gpt_5_bedrock_models_do_not_include_availability_nux_or_upgrade() {
+        let catalog = static_model_catalog();
+
+        for model in catalog.models {
+            assert_eq!((model.availability_nux, model.upgrade), (None, None));
+        }
+    }
+
     #[test]
     fn gpt_5_6_bedrock_models_clone_gpt_5_5_config_with_max_reasoning_effort() {
         let catalog = static_model_catalog();