git: improve file path validation in add operation

Add validation to ensure file paths are within repository boundaries
before staging. This prevents potential issues with relative paths
and improves overall robustness of the git_add function.

src/git/src/mcp_server_git/server.py

@@ -132,6 +132,14 @@ def git_add(repo: git.Repo, files: list[str]) -> str:
     if files == ["."]:
         repo.git.add(".")
     else:
+        # Validate paths are within repository before adding
+        for file in files:
+            try:
+                repo.git.check_attr('-a', file)
+            except git.exc.GitCommandError as e:
+                if 'outside repository' in str(e):
+                    raise ValueError(f"Path '{file}' is outside repository")
+                raise
         repo.index.add(files)
     return "Files staged successfully"