feat: use provider defaults for memory models

[celia-oai]

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

[jif-oai]

Ok after my comments