[ty] Modernize setup of union types in mdtests (#25291)

Remember when we wrote `x = 1 if flag else None` because we couldn't
infer types from annotated parameters and/or couldn't understand union
type annotations? This PR modernizes our mdtests to simply use annotated
function parameters in those cases (I manually rejected some rewrites
when the new style was less clear, for example when the generated union
type itself was hard to spell).

The motivation for this is to set a better precedent for agents, as I've
seen them repeating that `x if flag else y` pattern, which is really
unnecessary, longer and harder to read in most cases.