ssh: use errors.As to detect PartialSuccessError wrapped in BannerError

The authentication loop in newServerConn used a direct type assertion
authErr.(*PartialSuccessError) to detect partial-success responses from
authentication callbacks. This silently fails when a callback returns a
*BannerError that wraps a *PartialSuccessError.

BannerError has an Err field and implements Unwrap(), making it a
standard Go error-wrapping type. The PartialSuccessError doc says it
"can be returned by any of the ServerConfig authentication callbacks"
-- the same callbacks that may also return BannerError. It is therefore
valid and expected for a callback to return:

&BannerError{
    Message: "First factor accepted; provide second factor.",
    Err:     &PartialSuccessError{Next: ServerAuthCallbacks{...}},
}

This allows a callback to simultaneously send a banner message to the
client and signal partial success requiring a further auth step.

With the old direct type assertion, when authErr is a *BannerError
wrapping a *PartialSuccessError, ok is false. The partial-success state
is lost: authFailures is incremented, authConfig is not updated to
partialSuccess.Next, and the client receives SSH_MSG_USERAUTH_FAILURE
instead of SSH_MSG_USERAUTH_PARTIAL_SUCCESS.

The banner-extraction code immediately above already handles this
pattern correctly using errors.As for *BannerError.

This change replaces the three direct *PartialSuccessError type
assertions with errors.As, matching that existing pattern:

1. Main auth loop (authErr path): the primary fix -- ensures
   partialSuccess.Next updates authConfig even when wrapped.
2. PublicKeyCallback cache, first check: ensures the "invalid library
   usage" detection works when wrapped.
3. PublicKeyCallback cache, isQuery check: ensures the public-key query
   response (SSH_MSG_USERAUTH_PK_OK) is sent correctly when the
   callback result wraps a PartialSuccessError.

A new test TestPartialSuccessErrorWrappedInBannerError is added to
server_multi_auth_test.go. It sets up a PasswordCallback returning
&BannerError{Err: &PartialSuccessError{...}}, verifies the client can
authenticate successfully in two password steps, and confirms the test
fails without the fix.

Updates golang/go#79809