agent: Improve initial file search quality (#29317)

This PR significantly improves the quality of the initial file search
that occurs when the model doesn't yet know the full path to a file it
needs to read/edit.

Previously, the assertions in file_search often failed on main as the
model attempted to guess full file paths. On this branch, it reliably
calls `find_path` (previously `path_search`) before reading files.

After getting the model to find paths first, I noticed it would try
using `grep` instead of `path_search`. This motivated renaming
`path_search` to `find_path` (continuing the analogy to unix commands)
and adding system prompt instructions about proper tool selection.

Note: I know the command is just called `find`, but that seemed too
general.

In my eval runs, the `file_search` example improved from 40% ± 10% to
98% ± 2%. The only assertion I'm seeing occasionally fail is "glob
starts with `**` or project". We can probably add some instructions in
that regard.

Release Notes:

- N/A

crates/assistant_tools/src/find_path_tool/description.md

@@ -0,0 +1,7 @@
+Fast file path pattern matching tool that works with any codebase size
+
+- Supports glob patterns like "**/*.js" or "src/**/*.ts"
+- Returns matching file paths sorted alphabetically
+- Prefer the `grep` tool to this tool when searching for symbols unless you have specific information about paths.
+- Use this tool when you need to find files by name patterns
+- Results are paginated with 50 matches per page. Use the optional 'offset' parameter to request subsequent pages.