feat: use provider defaults for memory models (#27129)

## Why

Memory startup used hardcoded OpenAI model slugs for extraction and
consolidation. That works for the default OpenAI-compatible path, but
provider-specific backends can require different model identifiers. In
particular, Amazon Bedrock should use its Bedrock model ID for these
background memory requests instead of the OpenAI `gpt-5.4-mini` /
`gpt-5.4` slugs.

## What Changed

- Added provider-owned preferred memory model methods alongside
`approval_review_preferred_model`.
- Updated memory extraction and consolidation to resolve their default
model through the active `ModelProvider`.
- Added Amazon Bedrock overrides so both memory stages use
`openai.gpt-5.4` through Bedrock’s provider-specific model ID.
- Kept explicit `memories.extract_model` and
`memories.consolidation_model` config overrides taking precedence.
- Added startup coverage for default OpenAI and Bedrock memory model
selection.

#closes #26288

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

@@ -68,6 +68,14 @@ impl ModelProvider for AmazonBedrockModelProvider {
         AMAZON_BEDROCK_GPT_5_4_MODEL_ID
     }
 
+    fn memory_extraction_preferred_model(&self) -> &'static str {
+        AMAZON_BEDROCK_GPT_5_4_MODEL_ID
+    }
+
+    fn memory_consolidation_preferred_model(&self) -> &'static str {
+        AMAZON_BEDROCK_GPT_5_4_MODEL_ID
+    }
+
     fn auth_manager(&self) -> Option<Arc<AuthManager>> {
         None
     }