[Filesystem] add append tools

Is your feature request related to a problem? Please describe.
Currently, if I want to append a new line of text to an existing .txt file using this tool, there only have wirte_file now.

Describe the solution you'd like
I would like a new tool to be added, named append_file or append_wirte_file. This function should perform the following actions:

Accept two main arguments: the path to the target text file and the string content to be appended. Write the provided string content to the end of the file.

Crucially, automatically add a newline character (\n or the appropriate OS-specific newline) after the appended content. This ensures that each call to this function adds the content on a new line, making subsequent appends also start on a fresh line.

Handle the closing of the file automatically and safely.

Describe alternatives you've considered
None.

Additional context
None.

Start by locating the existing write_file entry point in the filesystem server and any related tests. Add the requested append_file tool with path and content arguments, newline-per-call behavior, and safe file closing; done means appends are preserved in order and the existing write behavior remains intact.