Are argumentsWithoutPositionalParameters and wasPositionalParameter more descriptive? (optional comment)

I changed names for more descriptive: argumentsWithoutProcedingParameters, parameterPreceding

Please use consistent bracing style of the surrounding code. For C# code, braces should be on their own line.

Fixed.

brace style as per comment above

Fixed.

I think you can inline the cast in the foreach loop: foreach (FunctionDefinitionAst functionDefinitionAst in functionDefinitionAsts)

Fixed.

!mandParams.Any() performs in general better than mandParams.Count() == 0 for IEnumerable types because the former stops on the first element but the latter enumerates over all elements. But in this case it is a List, therefore we can even just use the property mandParams.Count == 0, which does not need to enumerate at all. I know, this code was already like that before, I am not criticising you but we should take the opportunity and make this faster for free.

Sure, fixed.

the variable $violationName = "PSAvoidUsingPositionalParameters" defined at the top should be re-used

Fixed.

why not -Be 1?

The problem is that the other violation appears PSAvoidTrailingWhitespace if I write the script like this:
<$sb=
{
Function Foo {
param(
[Parameter(Mandatory=$true,Position=1)] $A,
[Parameter(Position=2)]$B,
[Parameter(Position=3)]$C)
}
Foo "a" "b" "c"
} >

The warning doesn't show up when the script looks like:
<$sb=
{
Function Foo {
param(
[Parameter(Mandatory=$true,Position=1)] $A,
[Parameter(Position=2)]$B,
[Parameter(Position=3)]$C)
}
Foo "a" "b" "c"}>

But I don't like its readability.

I am not sure if PSAvoidTrailingWhitespace is an expected behaviour in here?

The trailing whitespace is because of the indentation of the last brace, if you were to put the brace to the very left the rule would start to appear. Therefore this is by design and expected because it would be the same as if the last line in a script had space characters. Asserting against the exact number is more important than minor optical issues. I would be ok to just call Invoke-ScriptAnalyzer with -ExcludeRule PSAvoidTrailingWhitespace instead as an alternative. You should then also use the -Be operators for the other assertions. I proposed in older PRs to filter out only the relevant rules using Invoke-ScriptAnalyzer -ScriptDefintion $scriptDefinition | Where-Object { $_.RuleName -eq 'PSAvoidUsingPositionalParameters' } but Jim did not like this idea either (and he is also not in favour of testing the error message itself).

I got rid of AvoidTrailingWhitespaces trigger and of testing error message.

Shouldn't this be moreThanTwoPositionalParameters since the rule is >=3?

Fixed.

I think this can all be written more simply:

var commandElementCollection = cmdAst.CommandElements;
if ( commandElementCollection.Count = 1 ) { return false; }
for(int i = 1; i < commandElementCollection.Length; i++) {
if ( commandElementCollection[i] isnot CommandParameterAst && commandElementCollection[i-1] isnot CommandParameterAst ) {
argumentsWithoutProcedingParameters++;
}
}

this should catch things like test-cmd a -a -b c,f d,z $a $b -q $() -e -f @() @{ one = 1 } -z "this is a test" "this is a test"
it doesn't validate that there may be missing parameter arguments, but it should catch any positional parameters

Fixed.