Fixes #54244

When the `read_file` tool is called with a path that points to a
directory instead of a file, it now returns a clear, actionable error
message telling the agent to use `list_directory` instead.

Previously the tool would fail with an unhelpful generic error. Now it
explicitly checks whether the path is a directory before attempting to
read it.

A test covering this case is also included.

Release Notes:

- Fixed `read_file` tool returning an unhelpful error when given a
directory path; it now suggests using `list_directory` instead.

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Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>