fix(local): add IsDir checks to Read/Write/Edit to prevent directory-as-file errors

[luchun19921229]

Problem

When a user passes a directory path to Local.Read(), Local.Write(), or Local.Edit(), the methods attempt to use the path as a file — opening, reading, or writing it — which produces confusing low-level errors (e.g. is a directory from the OS) or silent failures instead of a clear explanation.

Changes

Added a guard check in each of the three methods that returns a clear error message when the target path is a directory rather than a file:

Read()

• Calls file.Stat() before info.Size() == 0 check
• Returns "path is a directory, not a file: <path>"

Write()

• Calls os.Stat(path) before MkdirAll/OpenFile
• Returns "path is a directory, not a file: <path>"

Edit()

• Calls os.Stat(path) before os.ReadFile(path)
• Returns "path is a directory, not a file: <path>"

Testing

• Run existing test suite to confirm no regressions
• Verify each method returns the directory error when given a directory path

[CLAassistant]

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[luchun19921229]

@cloudwego/eino-maintainers 请 review 这个 PR。CI 全绿，改动很简单：在 adk/backend/local/local.go 的 Read/Write/Edit 三个方法里加了 IsDir() 守卫，防止目录被误当成文件操作，给出清晰错误信息。