test(bf-5dopl): update unmapped glyph assertion message in resolver.rs

Updates the assertion in test_resolve_level2_unmapped_code to use the new
diagnostic message format designed in bf-w1o10.

- Changed bare assert!(result.is_failure()) to assert!(result.is_failure(), "...")
- Added structured message with Expected/Found/Why this matters sections
- Clarifies that codes above 0x7F in StandardEncoding are unmapped per build/unmapped-glyph-names.json

This completes the assertion message update task. All other unmapped glyph
assertions in unmapped.rs, encoding.rs, and test files were already updated
with the new format.

Closes bf-5dopl. All unmapped glyph tests pass (unmapped, encoding, resolver, cmap).

crates/pdftract-core/src/font/resolver.rs

@@ -870,7 +870,13 @@ mod tests {
         // Most codes in StandardEncoding are unmapped above 0x7F
         let encoding = FontEncoding::new(Some(NamedEncoding::Standard));
         let result = resolve_level2(&[0x80], Some(&encoding));
-        assert!(result.is_failure());
+        assert!(
+            result.is_failure(),
+            "Level 2 resolution should fail for unmapped character codes. \
+             Expected: result.is_failure() == true. \
+             Found: false. \
+             Why this matters: Most codes in StandardEncoding above 0x7F are unmapped and should fail Level 2 (encoding + AGL) resolution per build/unmapped-glyph-names.json filtering."
+        );
     }
 
     #[test]