bug(postgrest): Where() ignores ! and unary minus on captured variables

[fresh55]

Found this while deleting inactive accounts. The ! gets lost and the active ones get deleted instead:

bool keepActive = true;
await client.Table<Account>().Where(x => x.IsActive == !keepActive).Delete();
// sends is_active=eq.True

Same with unary minus:

int threshold = 100;
client.Table<Account>().Where(x => x.Balance < -threshold)
// sends balance=lt.100

HandleUnaryExpression never checks NodeType, so Not and Negate are treated like Convert.