fix(lsp): section-range termination is level-aware (caption audit follow-up)

The post-captions audit found find_section_range terminated a generated
section only at the next *level-1* heading. With a custom non-1
*_header_level (e.g. a level-2 Generated Tags index) a following same-level
section could be absorbed into the replaced span. Terminate at the next
heading of level <= the matched section's level instead, which keeps the
deeper `level+1` sub-groups inside while stopping at a sibling/parent.

Test: find_section_range_stops_at_same_level_sibling. 0 failures, clippy clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/nuwiki-lsp/tests/link_health.rs

@@ -305,6 +305,23 @@ fn build_toc_text_with_no_headings_is_just_the_heading() {
     assert_eq!(out, "= Contents =\n");
 }
 
+#[test]
+fn find_section_range_stops_at_same_level_sibling() {
+    // A non-level-1 generated section (e.g. a level-2 tags index) must not
+    // swallow the following same-level section when regenerated.
+    let src = "== Foo ==\n- a\n== Bar ==\n- b\n";
+    let ast = parse(src);
+    let (start, end) = ops::find_section_range(&ast, "Foo").expect("section found");
+    let bar_off = src.find("== Bar ==").unwrap();
+    assert_eq!(start.offset, 0);
+    assert!(
+        end.offset <= bar_off,
+        "section absorbed the sibling: end={} bar={}",
+        end.offset,
+        bar_off
+    );
+}
+
 #[test]
 fn captions_honour_custom_header_and_level() {
     // toc_header="Table of Contents", level 2 → `== Table of Contents ==`.