feat: switch WebSearch to the built-in Exa MCP-backed implementation and remove search-model configuration

[wgqqqqq]

Summary

This PR makes WebSearch the default path for web access and removes the old
search-model configuration flow from settings.

Changes

• disable WebFetch by default in Cowork mode
• keep WebSearch as the default web tool
• switch WebSearch to the built-in Exa MCP-backed implementation
• remove the dedicated search_enhanced model category from model settings
• remove search-related default model selection and capability config from the
  settings UI
• clean up related i18n entries in both en-US and zh-CN

Why

Using WebFetch as a default web tool is too heavy for general web research
and can easily bloat context when full page content is returned.

WebSearch is a better default for retrieval-oriented workflows, and moving
search to a built-in implementation also makes the product configuration
simpler by removing the need for a separate search-model setup.

Impact

• new default mode configs will prefer WebSearch over WebFetch
• search model setup is no longer exposed in settings
• web search behavior is now backed by the built-in Exa MCP integration