When VS Code closes, the STDIN stream closes and readline() returns
empty bytes (b''). Previously this was incorrectly treated as an
empty line separator, causing an infinite loop with 100% CPU usage.

This fix:
- Detects EOF in get_headers() by checking for b'' and raising EOFError
- Handles EOFError in all three places that call get_headers():
  - The main loop
  - handle_response()
  - custom_input()
- Exits gracefully with sys.exit(0) when EOF is detected

The key insight is distinguishing between:
- EOF: readline() returns b'' (empty bytes)
- Empty line: readline() returns b'\r\n' or b'\n' (newline bytes)

Also added comprehensive unit tests to verify the fix.