fix: handle ActiveRecord::RecordInvalid in ssh_creds when workspace is reset

[4ravind-b]

Fixes #21480

When workspace -D is run while a session is active, the session's
database record gets deleted but the session itself stays alive. When
post/multi/gather/ssh_creds tries to save credentials, it fails with
ActiveRecord::RecordInvalid: Session can't be blank and crashes the module.

This fix adds a rescue for ActiveRecord::RecordInvalid so the module
prints a warning and continues instead of crashing.

Verification

• Start msfconsole
• Get a session on target
• Run post/multi/gather/ssh_creds — works fine
• Run workspace -D
• Run post/multi/gather/ssh_creds again
• Verify module prints a warning instead of crashing
• Verify files are still downloaded successfully

[4ravind-b]

Verified on Metasploitable2.

Before this fix: Running the module after deleting the workspace with workspace -D caused the module to crash and print a full stack trace.

After this fix:

msf post(multi/gather/ssh_creds) > workspace -D
[*] Deleted workspace: default
[*] Recreated the default workspace

msf post(multi/gather/ssh_creds) > run
[*] Finding .ssh directories
[*] Looting 3 .ssh directories
[*] Looting /home/msfadmin/.ssh directory
[+] Downloaded /home/msfadmin/.ssh/authorized_keys -> ...
[+] Downloaded /home/msfadmin/.ssh/id_rsa -> ...
[!] Could not save credential to database: Validation failed: Session can't be blank
[+] Downloaded /home/msfadmin/.ssh/id_rsa.pub -> ...
[*] Looting /home/user/.ssh directory
[+] Downloaded /home/user/.ssh/id_dsa -> ...
[!] Could not save credential to database: Validation failed: Session can't be blank
[*] Looting /root/.ssh directory
[+] Downloaded /root/.ssh/authorized_keys -> ...
[*] Post module execution completed

The module no longer crashes. Instead, it prints a warning when credential storage fails and continues processing the remaining files. All SSH files are successfully downloaded, post-module execution completes normally, and the session remains active.