test(auth): add integration tests for password-based authentication at connection time

[currantw]

Describe the feature

There are currently no integration tests that verify connecting to a password-protected server at connection time using ServerCredentials. The existing UpdateConnectionPasswordTests only test the dynamic password update flow (connecting without a password first, then setting/clearing one). This leaves the initial AUTH handshake during connection establishment untested.

The ServerCredentials class has two password-based constructors:

• ServerCredentials(string password) — password only (authenticates as default user)
• ServerCredentials(string? username, string password) — username + password (authenticates as a specific ACL user)

Neither is exercised in any integration test.

Use Case

Password-based authentication is the most common authentication mode for Valkey/Redis. Without tests covering initial connection auth, regressions in the AUTH handshake (or credential passing to glide-core) could go undetected.

Proposed Solution

Add a new integration test class (e.g., PasswordAuthTests) with the following test cases, each parameterized for both standalone and cluster modes:

1. Connect with correct password — succeeds
2. Connect with wrong password — fails with expected exception
3. Connect without credentials when server requires a password — fails with expected exception
4. Connect with correct username + password — succeeds
5. Connect with correct username + wrong password — fails with expected exception
6. Connect with wrong username + correct password — fails with expected exception

Acknowledgements

• I may be able to implement this feature request
• This feature might cause a breaking change

Related

• tests/Valkey.Glide.IntegrationTests/UpdateConnectionPasswordTests.cs — related but only covers dynamic password updates
• tests/Valkey.Glide.IntegrationTests/IamAuthTests.cs — tests IAM auth path, not password auth
• sources/Valkey.Glide/ServerCredentials.cs — the class under test

Additional Information

• The test server utilities (ServerFixture, Server) already support SetPasswordAsync / ClearPasswordAsync, which can be used to configure the server before connecting.
• ACL user tests will need to create/remove ACL users on the server (e.g., via ACL SETUSER / ACL DELUSER).
• Tests that mutate server auth state should disable parallel execution to avoid interfering with other tests.