fix: Prevent WriteFileTool from writing to directory paths

- Enhances WriteFileTool validation to check if the target file_path is an existing directory.
- If it is, the tool now returns a validation error "Path is a directory, not a file: <filePath>", preventing the attempt to write.
- This proactive check avoids underlying file system errors that would occur if fs.writeFileSync were called on a directory path, which could lead to console errors.
- Test cases have been updated to reflect this stricter validation.

Fixes https://b.corp.google.com/issues/418348176

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