Strengthen type-mismatch test to pin Money diagnostic wording

[Copilot]

What this changes

The existing test only asserted that a TYPE_MISMATCH diagnostic code existed, which allowed regressions in rendered diagnostic wording. This updates that single test to also require that the rendered message mentions Money.

• Scope

  • Touches only crates/deed-typeck/tests/checking.rs
  • Modifies only an_operator_with_two_known_sides_is_checked

• Test assertion strengthening

  • Keeps the existing code-level assertion:
    • codes_of(&checked.diagnostics).contains(&codes::TYPE_MISMATCH)
  • Adds rendered-text pinning for the operand type:
    • message must contain either found \Money`or ``Money` ``

• Snippet

  let text = rendered(&sources, &checked.diagnostics);
  assert!(text.contains("found `Money`") || text.contains("`Money`"), "{text}");

Why this way

Code-only checks verify classification, not user-facing clarity. Pinning Money in rendered output makes the test guard both type-check behavior and diagnostic usefulness while staying tolerant to minor phrasing variation.

What it still does not do

It does not fully pin the complete diagnostic sentence; it only requires inclusion of the relevant type name (Money) to avoid over-coupling to exact formatting.

Checks

• cargo fmt --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace
• cargo run -p deed-cli -- check examples/ and test examples/, if this touches the compiler
• A design document under design/ updated in this PR, if this changes behaviour one of
  them describes. A design that lags the code is worse than no design.
• Tests added for the thing that was wrong, not only for the thing that is now right. A
  guard test that only passes values the guard accepts proves nothing, and that is how
  the worst bug here went unnoticed.

AI assistance: Used to draft and implement the test update.

Original prompt

In crates/deed-typeck/tests/checking.rs, strengthen the existing test an_operator_with_two_known_sides_is_checked.

It currently only checks that TYPE_MISMATCH is among the codes. Pin rendered wording.

Change it to:

#[test]
fn an_operator_with_two_known_sides_is_checked() {
    let (sources, checked) = check_source(
        "module a\n\nrecord Money { units: Int }\n\nfn f(m: Money, n: Money) -> Int { m + n }\n",
    );
    assert!(
        codes_of(&checked.diagnostics).contains(&codes::TYPE_MISMATCH),
        "{}",
        rendered(&sources, &checked.diagnostics)
    );
    let text = rendered(&sources, &checked.diagnostics);
    assert!(text.contains("found `Money`") || text.contains("`Money`"), "{text}");
}

Only edit that one test. No production code.