feat(lsp): wire diagnostic.link_severity through from client config

The broken-link diagnostic severity was already consulted by
collect_diagnostics, but Config.diagnostic was hardcoded to the default and
never populated from initializationOptions/didChangeConfiguration. Parse a
client-supplied `diagnostic.link_severity` ('off'|'hint'|'warn'|'error',
case-insensitive) into LinkSeverity and apply it in both from_init_params and
apply_change. Partial config updates that omit `diagnostic` preserve the
existing severity rather than resetting it.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/nuwiki-lsp/tests/index_and_config.rs

@@ -319,6 +319,54 @@ fn raw_wiki_parses_every_new_key() {
     assert!(w.auto_toc);
 }
 
+#[test]
+fn link_severity_parses_from_client_config() {
+    for (input, expected) in [
+        ("off", LinkSeverity::Off),
+        ("hint", LinkSeverity::Hint),
+        ("warn", LinkSeverity::Warning),
+        ("warning", LinkSeverity::Warning),
+        ("error", LinkSeverity::Error),
+        ("ERROR", LinkSeverity::Error),
+    ] {
+        let cfg = config_from_json(json!({
+            "wiki_root": "/tmp/w",
+            "diagnostic": { "link_severity": input },
+        }));
+        assert_eq!(
+            cfg.diagnostic.link_severity, expected,
+            "input {input:?} should map to {expected:?}"
+        );
+    }
+}
+
+#[test]
+fn link_severity_invalid_or_absent_falls_back_to_default() {
+    // Unknown string → default (Warning).
+    let bad = config_from_json(json!({
+        "wiki_root": "/tmp/w",
+        "diagnostic": { "link_severity": "loud" },
+    }));
+    assert_eq!(bad.diagnostic.link_severity, LinkSeverity::Warning);
+
+    // No diagnostic key at all → default.
+    let none = config_from_json(json!({ "wiki_root": "/tmp/w" }));
+    assert_eq!(none.diagnostic.link_severity, LinkSeverity::Warning);
+}
+
+#[test]
+fn apply_change_preserves_severity_on_partial_update() {
+    let mut cfg = config_from_json(json!({
+        "wiki_root": "/tmp/w",
+        "diagnostic": { "link_severity": "error" },
+    }));
+    assert_eq!(cfg.diagnostic.link_severity, LinkSeverity::Error);
+
+    // A later update that omits `diagnostic` must not reset severity.
+    cfg.apply_change(&json!({ "wiki_root": "/tmp/w2" }));
+    assert_eq!(cfg.diagnostic.link_severity, LinkSeverity::Error);
+}
+
 #[test]
 fn vim_flat_and_neovim_wrapped_payloads_desugar_identically() {
     // Parity guard for the two client shapes:
@@ -332,6 +380,7 @@ fn vim_flat_and_neovim_wrapped_payloads_desugar_identically() {
         "file_extension": ".md",
         "syntax": "vimwiki",
         "log_level": "debug",
+        "diagnostic": { "link_severity": "error" },
         "wikis": [
             {
                 "name": "personal",
@@ -347,6 +396,11 @@ fn vim_flat_and_neovim_wrapped_payloads_desugar_identically() {
     let flat = config_from_json(inner.clone());
     let wrapped = config_from_json(json!({ "nuwiki": inner }));
 
+    assert_eq!(
+        flat.diagnostic.link_severity,
+        wrapped.diagnostic.link_severity
+    );
+    assert_eq!(flat.diagnostic.link_severity, LinkSeverity::Error);
     assert_eq!(flat.wikis.len(), wrapped.wikis.len());
     for (a, b) in flat.wikis.iter().zip(wrapped.wikis.iter()) {
         assert_eq!(a.name, b.name);