Add explicit binascii.Error catch and bad-padding test

binascii.Error is already a subclass of ValueError, but listing it
explicitly makes the intent clear. Added test for incorrect base64
padding (e.g. "Basic abc").

esphome/dashboard/web_server.py

@@ -123,7 +123,7 @@ def is_authenticated(handler: BaseHandler) -> bool:
                 try:
                     auth_decoded = base64.b64decode(auth_header[6:]).decode()
                     username, password = auth_decoded.split(":", 1)
-                except (ValueError, UnicodeDecodeError):
+                except (binascii.Error, ValueError, UnicodeDecodeError):
                     return False
                 return settings.check_password(username, password)
         return handler.get_secure_cookie(AUTH_COOKIE_NAME) == COOKIE_AUTHENTICATED_YES

tests/dashboard/test_web_server.py

@@ -1707,6 +1707,14 @@ def test_is_authenticated_malformed_base64(
     assert web_server.is_authenticated(handler) is False
 
 
+def test_is_authenticated_bad_base64_padding(
+    mock_auth_settings: MagicMock,
+) -> None:
+    """Test that incorrect base64 padding (binascii.Error) returns False."""
+    handler = _make_auth_handler("Basic abc")
+    assert web_server.is_authenticated(handler) is False
+
+
 def test_is_authenticated_invalid_utf8(
     mock_auth_settings: MagicMock,
 ) -> None: