Refactor shell management and add SFTP integration tests

[OutOfBedlam]

This pull request introduces improvements to SSH server handling and testing, particularly focusing on SFTP support and command execution reliability. It also removes unused shell management code from the server management module. The most significant changes are grouped as follows:

SSH Server Improvements:

• The SFTP handler in sshd now sets the working directory explicitly using the first entry in FileDirs, improving SFTP session consistency.
• In the SSH command handler, output stream management is now handled using a WaitGroup instead of a sync.Once closure, ensuring all output streams are properly closed before the session ends. This prevents premature session closure and potential race conditions. [1] [2] [3] [4] [5]
• The command exit code handling logic is updated for better reliability, ensuring the correct exit code is sent to the SSH client.

Testing Enhancements:

• Adds a comprehensive SFTP integration test (TestSSHSftp) to verify directory creation, file upload, file stat, directory listing, and file download functionality over SFTP.
• Introduces a new SSH command test suite (TestSSHCommands and SSHTestCommand) to validate command execution and output matching, improving test coverage for SSH interactions.
• Test dependencies are updated to include the SFTP package and necessary imports. [1] [2]

Code Cleanup:

• Removes unused shell management API types and functions (ListShell, AddShell, DelShell, and related types) from svrmgmt.go, simplifying the server management codebase. [1] [2]

[codecov]

Codecov Report

❌ Patch coverage is 84.61538% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.03%. Comparing base (afd582a) to head (16364a0).

Files with missing lines	Patch %	Lines
mods/server/ssh.go	84.61%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #417      +/-   ##
==========================================
+ Coverage   63.89%   64.03%   +0.14%     
==========================================
  Files         353      353              
  Lines       68359    68307      -52     
==========================================
+ Hits        43675    43743      +68     
+ Misses      20080    19946     -134     
- Partials     4604     4618      +14     

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