Support complex numeric expression arguments in scalar function predicates pushdown

### Is your feature request related to a problem or challenge?

Currently, scalar function pushdown (e.g. `isnan`) only supports bare column references. Complex expressions (so far only unary expressions are supported) like are silently dropped.
This issue tracks adding support for more complex numeric expressions as arguments to scalar functions, and potentially additional scalar functions beyond `isnan`.

### Describe the solution you'd like

_No response_

### Willingness to contribute

I can contribute to this feature independently

Start by tracing the scalar function pushdown implementation and its existing isnan handling, then inspect how unary expressions are represented and filtered. Done means complex numeric expression arguments are no longer silently dropped, with coverage for the supported cases and any added scalar functions.

Written by the indexing model from the issue text.