Fix trailing backslash in shell registry operations

[EclipseAditya]

registry_enumkeys and other shell-based registry functions silently fail when the key path ends with a trailing backslash (\). This is because normalize_key preserves the trailing backslash, which then gets interpolated into a cmd.exe command like:

cmd.exe /c reg query "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\"

The \" at the end is interpreted by cmd.exe as an escaped double-quote, producing a malformed command. Meterpreter sessions are unaffected because they use the Windows API directly, which tolerates trailing backslashes.

This caused exploit/windows/persistence/registry (and similar modules) to incorrectly report "System does not have powershell" on shell sessions while working correctly on meterpreter sessions.

The fix:

• Strip trailing backslash in normalize_key via .chomp("\\") — this protects all 10+ shell registry functions at the normalization layer
• Remove trailing backslashes from all affected module callers (defense in depth)
• Add RSpec tests for normalize_key and split_key

Fixes #20899

Verification

• Start msfconsole
• Obtain a shell session on a Windows target (e.g., windows/x64/shell/reverse_tcp)
• use exploit/windows/persistence/registry
• set SESSION <shell_session_id>
• check
• Verify the output is The target is vulnerable. Registry writable (not System does not have powershell)
• Obtain a meterpreter session on the same target
• set SESSION <meterpreter_session_id>
• check
• Verify the output is still The target is vulnerable. Registry writable (no regression)
• Run bundle exec rspec spec/lib/msf/core/post/windows/registry_spec.rb
• Verify all 19 examples pass, 0 failures