fix nondeterministic diagnostic message ordering in tests

Some cases:

```
Input file: fixtures/ty_check/method_selection/require_explicit_import.fe
──────────────────────────────────────────────────────────────────────────
Expression: diags
──────────────────────────────────────────────────────────────────────────
-old snapshot
+new results
────────────┬─────────────────────────────────────────────────────────────
3 3 │ 2 │ x.foo()
4 4 │ │ ^^^
5 5 │ │ │
6 6 │ │ consider importing one of the following traits into the scope to resolve the ambiguity
7 │- │ `use require_explicit_import::inner::Bar`
8 │- │ `use require_explicit_import::inner::Foo`
7 │+ │ `use require_explicit_import::inner::Foo`
8 │+ │ `use require_explicit_import::inner::Bar`
```

```
Input file: fixtures/ty_check/method_selection/ambiguous_trait_def.fe
──────────────────────────────────────────────────────────────────────────
Expression: diags
──────────────────────────────────────────────────────────────────────────
-old snapshot
+new results
────────────┬─────────────────────────────────────────────────────────────
3 3 │ 38 │ let y = f.foo()
4 4 │ │ ^^^
5 5 │ │ │
6 6 │ │ `foo` is ambiguous
7 │+ │ candidate: `Trait2::foo`
7 8 │ │ candidate: `Trait1::foo`
8 │- │ candidate: `Trait2::foo`
```

Start with the diagnostic snapshots under fixtures/ty_check/method_selection/require_explicit_import.fe and ambiguous_trait_def.fe, then trace the code that produces the candidate and trait-import diagnostic lists. Run the affected type-checking fixture tests and make their output ordering deterministic so both snapshots pass consistently.

Written by the indexing model from the issue text.