Improve script tool description and add lines iterator to Lua file objects (#26529)

Release Notes:

- N/A

---------

Co-authored-by: Agus Zubiaga <hi@aguz.me>

crates/scripting_tool/src/scripting_tool_description.md

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+Evaluates the given Lua script in an interpreter with access to the Lua standard library. The tool returns the scripts output to stdout and any error that may have occurred.
+
+Use this tool to explore the current project and edit the user's codebase or operating system as requested.
+
+Additional functions provided:
+
+```lua
+--- Search for matches of a regular expression in files.
+-- @param pattern The regex pattern to search for (uses Rust's regex syntax)
+-- @return An array of tables with 'path' (file path) and 'matches' (array of matching strings)
+-- @usage local results = search("function\\s+\\w+")
+function search(pattern)
+  -- Implementation provided by the tool
+end
+
+--- Generates an outline for the given file path, extracting top-level symbols such as functions, classes, exports, and other significant declarations. This provides a structural overview of the file's contents.
+-- @param path
+function outline(path)
+  -- Implementation provided by the tool
+end
+```