chore(workspace): satisfy clippy --all-targets in test code

Five test-only lints surfaced by --all-targets were blocking CI under
-D warnings: a useless vec! in a leaderboard sort test, a
field_reassign_with_default in tuning tests, and three
assertions_on_constants in card_plugin sanity tests. The constant
assertions are now wrapped in const blocks so they run at compile time;
the runtime-formatted values were dropped from their messages because
const-block assert messages must be string literals.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

solitaire_server/src/leaderboard.rs

@@ -194,7 +194,7 @@ mod tests {
 
     #[test]
     fn leaderboard_entries_sorted_by_score_descending() {
-        let mut entries = vec![
+        let mut entries = [
             entry("Charlie", Some(1_200)),
             entry("Alice",   Some(8_000)),
             entry("Bob",     Some(3_500)),