fix: decode help-banner test subprocess output as UTF-8

Rich renders the banner panel with box-drawing characters (╭, ╮, │, etc.)
that cp1252 cannot decode. On Windows, subprocess.run(..., text=True) uses
cp1252 by default, so decoding the child stdout raises UnicodeDecodeError
and subprocess sets result.stdout to None — breaking the assertion with a
misleading "argument of type 'NoneType' is not iterable".

Pass encoding="utf-8" explicitly so the test passes on every platform.

Author: mohamedashrraf222

tests/test_help_banner.py

@@ -4,15 +4,21 @@
 
 def test_help_displays_logo() -> None:
     result = subprocess.run(
-        [sys.executable, "-c", "from codeflash.main import main; main()", "--help"], capture_output=True, text=True
+        [sys.executable, "-c", "from codeflash.main import main; main()", "--help"],
+        capture_output=True,
+        text=True,
+        encoding="utf-8",
     )
     assert result.returncode == 0
     assert "codeflash.ai" in result.stdout
 
 
 def test_help_short_flag_displays_logo() -> None:
     result = subprocess.run(
-        [sys.executable, "-c", "from codeflash.main import main; main()", "-h"], capture_output=True, text=True
+        [sys.executable, "-c", "from codeflash.main import main; main()", "-h"],
+        capture_output=True,
+        text=True,
+        encoding="utf-8",
     )
     assert result.returncode == 0
     assert "codeflash.ai" in result.stdout