PR Summary

Improves the argument completion for attributes so it can work in all scenarios I can think of.
Today the attribute completion only works reliably when you have a variable to bind the attribute to, as the parser throws away the AttributeAst if it can't find a variable. I have fixed this by adding the parsed AttributeAsts to the ErrorExpression that would be created in this case.
Inside a param block the parser already keeps the parsed attributes like this so I have updated the completion code to also handle how the attributes are stored for incomplete parameters inside the param block.

PR Context

Fixes#25111

Regression comes from#17902

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