Strengthen contract-clause condition diagnostic test coverage

[Copilot]

What this changes

The contract_clauses_must_be_conditions test only asserted TYPE_MISMATCH, which allowed diagnostic wording regressions to slip through. This updates that single test to also assert rendered output contains condition/type expectation language.

• Scope (single-test change only)

  • Updated crates/deed-typeck/tests/checking.rs::contract_clauses_must_be_conditions
  • Kept the existing diagnostic code assertion unchanged
  • Added rendered-diagnostic text assertion using rendered(&sources, &checked.diagnostics)

• Guard added

  • Asserts the rendered message includes at least one of: "Bool", "condition", or "expected".

let text = rendered(&sources, &checked.diagnostics);
assert!(text.contains("Bool") || text.contains("condition") || text.contains("expected"), "{text}");

Why this way

Code-only assertions verify classification, not message quality. This adds a minimal, stable wording guard without overfitting to one exact sentence.

What it still does not do

It does not pin an exact full diagnostic string; it enforces key semantic wording only.

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.

Original prompt

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

Current test only asserts TYPE_MISMATCH. Pin the rendered wording.

Change it to:

#[test]
fn contract_clauses_must_be_conditions() {
    let (sources, checked) = check_source("module a\n\nfn f(n: Int) -> Int\n  where n,\n{ n }\n");
    assert_eq!(codes_of(&checked.diagnostics), vec![codes::TYPE_MISMATCH]);
    let text = rendered(&sources, &checked.diagnostics);
    assert!(text.contains("Bool") || text.contains("condition") || text.contains("expected"), "{text}");
}

Only edit that one test. No production code.