fix: don't drop cause-chain when rendering related errors as nested

Previously, using the show_related_errors_as_nested feature introduced
in 771a075, any cause-chain of those nested errors would be dropped,
so the information would get lost.

src/handlers/graphical.rs

@@ -504,7 +504,10 @@ impl GraphicalReportHandler {
 
                     let mut inner_renderer = self.clone();
                     inner_renderer.footer = None;
-                    inner_renderer.with_cause_chain = false;
+                    // Each related error is an independent diagnostic whose cause
+                    // chain has not been flattened, so keep printing it to avoid
+                    // dropping the source chain and its details.
+                    inner_renderer.with_cause_chain = true;
                     inner_renderer.termwidth -= rest_indent.width();
                     inner_renderer.render_report_inner(&mut inner, rel, src)?;
 

tests/test_diagnostic_source_macro.rs

@@ -345,6 +345,16 @@ fn test_display_related_errors_as_nested() {
   × A multi-error happened
   ├─▶   × A multi-error happened
   │     ├─▶   × A nested error happened
+  │     │     ├─▶   × TestError
+  │     │     │
+  │     │     ╰─▶   × A complex error happened
+  │     │            ╭────
+  │     │          1 │ This is another error
+  │     │            ·    ──┬─
+  │     │            ·      ╰── here
+  │     │            ╰────
+  │     │           help: You should fix this
+  │     │
   │     │      ╭────
   │     │    1 │ right here
   │     │      ·       ──┬─