fix(conversation): improve workspace search

[CottChen]

Description

Companion backend changes for iOfficeAI/AionUi#3539.

This PR improves workspace search used by the project panel:

• Search files recursively instead of only direct children.
• Use ignore::WalkBuilder so .gitignore and git excludes are respected.
• Skip expensive directories by default, including .git, node_modules, target, and dist.
• Still allow searching inside an ignored or normally skipped directory when that directory is selected explicitly as the search root.
• Add search modes: all, name, and content.
• Return match metadata so the frontend can prioritize filename matches and show file/content-block counts.

Why

The AionUi project panel now exposes scoped search, search modes, and search result counts. Those UI behaviors depend on the backend returning recursive, mode-aware search results with match-kind metadata. The default pruning avoids accidentally walking dependency/build directories while keeping explicit scoped searches possible.

Tradeoffs

The search remains bounded by the existing result limit and is optimized for interactive project-panel usage rather than full indexing. This keeps the implementation simple and avoids background index state, at the cost of doing a fresh walk per query.

Local Checks

• cargo fmt --all --check
• cargo test -p aionui-conversation browse_workspace_search_
• cargo clippy -p aionui-conversation -- -D warnings