Add Code Symbols tool (#27733)

Lets you get all the code symbols in the project (like the Code Symbols
panel) or in a particular file (like the Outline panel), optionally
paginated and filtering results by regex. The tool gives the files,
lines, and numbers of all of these, which means they can be used in
conjunction with the read file tool to read subsets of large files
without having to open the entire large file and poke around in it.

<img width="621" alt="Screenshot 2025-03-29 at 12 00 21 PM"
src="https://github.com/user-attachments/assets/d78259d7-2746-44c0-ac18-2e21f2505c0a"
/>

Release Notes:

- N/A

crates/assistant_tools/src/code_symbols_tool/description.md

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+Returns either an outline of the public code symbols in the entire project (grouped by file) or else an outline of both the public and private code symbols within a particular file.
+
+When a path is provided, this tool returns a hierarchical outline of code symbols for that specific file.
+When no path is provided, it returns a list of all public code symbols in the project, organized by file.
+
+You can also provide an optional regular expression which filters the output by only showing code symbols which match that regex.
+
+Results are paginated with 2000 entries per page. Use the optional 'offset' parameter to request subsequent pages.
+
+Markdown headings indicate the structure of the output; just like
+with markdown headings, the more # symbols there are at the beginning of a line,
+the deeper it is in the hierarchy.
+
+Each code symbol entry ends with a line number or range, which tells you what portion of the
+underlying source code file corresponds to that part of the outline. You can use
+that line information with other tools, to strategically read portions of the source code.
+
+For example, you can use this tool to find a relevant symbol in the project, then get the outline of the file which contains that symbol, then use the line number information from that file's outline to read different sections of that file, without having to read the entire file all at once (which can be slow, or use a lot of tokens).
+
+<example>
+# class Foo [L123-136]
+## method do_something(arg1, arg2) [L124-126]
+## method process_data(data) [L128-135]
+# class Bar [L145-161]
+## method initialize() [L146-149]
+## method update_state(new_state) [L160]
+## private method _validate_state(state) [L161-162]
+</example>
+
+This example shows how tree-sitter outlines the structure of source code:
+
+1. `class Foo` is defined on lines 123-136
+   - It contains a method `do_something` spanning lines 124-126
+   - It also has a method `process_data` spanning lines 128-135
+
+2. `class Bar` is defined on lines 145-161
+   - It has an `initialize` method spanning lines 146-149
+   - It has an `update_state` method on line 160
+   - It has a private method `_validate_state` spanning lines 161-162