Fix fork handling and pipe in executor.c

[ronak-15]

Changes Made

• Updated execute_commands in executor.c to handle:
  • Safe fork() error handling
  • Pipe handling between commands

Testing Done

• Manual tests:
  • ls
  • ls | grep ".c"
  • echo "hello" > test.txt and cat < test.txt
  • echo "append" >> test.txt
• Verified forks, pipes, and redirection work as expected.
  But after testing I have deleted the files.

Notes

• This change does not affect built-in commands.
• No new dependencies added.