tamer: diagnose::report: Restore source line preview for invalid UTF-8

This was removed in a previous commit while working on simplifying the
implementation, with the hope of returning to it once things were in a
better place.  They are, so let's bring it back.

DEV-12151

tamer/src/diagnose/report.rs

@@ -386,6 +386,11 @@ where
                             }),
                         ]);
                     } else {
+                        body.extend(vec![
+                            SectionLine::SourceLinePadding,
+                            SectionLine::SourceLine(srcline.into()),
+                            SectionLine::SourceLinePadding,
+                        ]);
                         body.extend(
                             label.map(|l| SectionLine::Footnote(level, l)),
                         );

tamer/src/diagnose/report/test/integration.rs

@@ -453,6 +453,7 @@ fn fallback_when_column_fails_to_resolve() {
     let ctx = Context::from("invalid/utf8");
 
     let span = ctx.span(4, 2);
+    let lossy = String::from_utf8_lossy(FILE_INVALID_UTF8);
 
     assert_report!(
             "column resolution failure",
@@ -460,6 +461,9 @@ fn fallback_when_column_fails_to_resolve() {
             format!("\
 error: column resolution failure
   --> invalid/utf8:1 bytes 4--6
+   |
+ 1 | {lossy}
+   |
    = error: an error we do not want to suppress
    = help: unable to calculate columns because the line is not a valid UTF-8 string
    = help: you have been provided with 0-indexed line-relative inclusive byte offsets