If file is too big, provide the outline and suggest a follow-up tool (#28158)

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src="https://github.com/user-attachments/assets/24b9c7d4-d3e2-4929-bca8-79db5b4e5748"
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Release Notes:

- The `read_files` tool now reads only the symbol outline files above a
certain size, to conserve context window space. Then it suggests that
the agent call `read_files` again with the relevant line ranges it saw
in the outline.

crates/assistant_tools/src/read_file_tool/description.md

@@ -1 +1,6 @@
 Reads the content of the given file in the project.
+
+If the file is too big to read all at once, and neither a start line
+nor an end line was specified, then this returns an outline of the
+file's symbols (with line numbers) instead of the file's contents,
+so that it can be called again with line ranges.