fix(parser): infer for loop items from iterable unions

[Alb-O]

Description

parse_for inferred the loop variable from the type of the iterable expression. That works for direct list, table, and range types, but fails for iterable unions returned by commands like skip.

For oneof<table, binary, list<any>>, the loop variable was typed as the whole iterable union instead of the yielded row or item type. With enforce-runtime-annotations enabled, assigning the yielded value to that loop variable raised a runtime type mismatch.

The parser now maps iterable types to their yielded types when inferring the loop variable type, including alternatives inside oneof.

example: filter output union

Code	let pending = ([a] | each {} | collect | skip 0) for item in $pending { print $item }
Before	Error: nu::shell::type_mismatch × Type mismatch. ╭─[source:1:17] 1 │ let pending = ([a] | each {} | collect | skip 0); for item in $pending { print $item } · ┬ ──┬─ · │ ╰── expected oneof<table, binary, list<any>>, got string · ╰── the value is a string ╰────
After	a

User-facing changes (Release notes)

• Fixed an issue in for loops where type of the looping variable was inferred incorrectly when iterating over a value/stream with a union type (oneof<list, table, binary>). This could manifest as a runtime type checking error with enforce-runtime-annotations enabled.

[fdncred]

Thanks!