#14043: fix: make '/model default' reset to configured default model

Fixes #13982 `/model default` should reset the current session override back to the configured default model. Before this change, `default` was treated like a literal model name, which could resolve to invalid IDs like `anthropic/default`. ### What changed - Added explicit handling for `default` (without a provider prefix) in model directive resolution. - It now maps to configured `defaultProvider/defaultModel` and clears the session override. - Added an e2e test for: 1) `/model openai/gpt-4.1-mini` 2) `/model default` 3) verify override is cleared. ### Validation - Ran targeted e2e test for the new flow and it passes.