Exhaustiveness checker accepts (_, _) but rejects equivalent concrete types in tuple match #4937

Description

When using match \exhaustive\ on a tuple of union types, the exhaustiveness checker accepts (_, _) as a final catch-all pattern but rejects (None, None) in the same position — even when None is the only remaining possibility for each tuple element.

Example that works

type A is String

actor Main
  new create(env: Env) =>
    let x: (A | None) = None
    let y: (A | None) = None

    match \exhaustive\ (x, y)
    | (let a: A, _) => env.out.print("1")
    | (_, let a: A) => env.out.print("2")
    | (_, _) => env.out.print("3")
    end

Example that fails

The only change is (None, None) instead of (_, _):

type A is String

actor Main
  new create(env: Env) =>
    let x: (A | None) = None
    let y: (A | None) = None

    match \exhaustive\ (x, y)
    | (let a: A, _) => env.out.print("1")
    | (_, let a: A) => env.out.print("2")
    | (None, None) => env.out.print("3")
    end

Error:
main.pony:9:5: match marked \exhaustive\ is not exhaustive
    match \exhaustive\ (x, y)
    ^

Expected behavior

Both versions should be accepted. After the first two patterns cover all cases where at least one element is A, the only remaining combination is (None, None). The checker should recognize that (None, None) covers the same remaining space as (_, _) in this context.

Environment

ponyc 0.61.0 on Linux x86_64.