WinVaultKeyring backend does not always accept case-insensitive usernames

[putradimas]

Describe the bug
The WinVaultKeyring backend does not always accept case-insensitive usernames, even when Win32Cred does.

The following function causes this inconsistency:

keyring/keyring/backends/Windows.py

Lines 102 to 110 in 38c0401

	def _resolve_credential(
	self, service: str, username: str | None
	) -> DecodingCredential | None:
	# first attempt to get the password under the service name
	res = self._read_credential(service)
	if not res or username and res['UserName'] != username:
	# It wasn't found so attempt to get it with the compound name
	res = self._read_credential(self._compound_name(username, service))
	return res

The code checks if the service exists in the Windows Credential Locker with an exact, case-sensitive match of the username (res['UserName'] != username). If it's not found, either because it doesn't exist or because the casing is different, it looks up the compound name {username}@{service}, which completely skips username comparison.

To Reproduce

> keyring set test USER
Password for 'USER' in 'test': 
> keyring get test USER
PASS
> keyring get test user
> keyring set test user
Password for 'user' in 'test': 
> keyring get test user
pass
> keyring get test USER
PASS
> keyring get test usER
PASS
> keyring set test usER
Password for 'usER' in 'test': 
> keyring get test usER
paSS
> keyring get test USER
pass
> keyring get test user
pass

Expected behavior
WinVaultKeyring should use consistent case sensitivity.

Environment

• OS: Windows

> pip list | Select-String "keyring"
keyring           25.7.0
> keyring --list-backends
keyring.backends.chainer.ChainerBackend (priority: -1)
keyring.backends.fail.Keyring (priority: 0)
keyring.backends.Windows.WinVaultKeyring (priority: 5)