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/src/config.rs

@@ -153,6 +153,21 @@ pub enum LinkSeverity {
     Error,
 }
 
+impl LinkSeverity {
+    /// Parse a client-supplied severity string. Case-insensitive;
+    /// accepts both `warn` and `warning`. Returns `None` for unknown
+    /// values so callers can fall back to the default.
+    pub fn parse(s: &str) -> Option<Self> {
+        match s.trim().to_ascii_lowercase().as_str() {
+            "off" => Some(Self::Off),
+            "hint" => Some(Self::Hint),
+            "warn" | "warning" => Some(Self::Warning),
+            "error" => Some(Self::Error),
+            _ => None,
+        }
+    }
+}
+
 impl WikiConfig {
     /// Synthetic default used when the client supplies neither
     /// `initializationOptions` nor a workspace root. `name` is empty;
@@ -360,7 +375,7 @@ impl Config {
 
         Config {
             wikis,
-            diagnostic: DiagnosticConfig::default(),
+            diagnostic: diagnostic_from_raw(&raw.diagnostic),
         }
     }
 
@@ -402,6 +417,11 @@ impl Config {
             }
             self.wikis = vec![wc];
         }
+        // Only touch diagnostic settings when the payload carries them, so a
+        // partial `didChangeConfiguration` doesn't reset severity to default.
+        if raw.diagnostic.is_some() {
+            self.diagnostic = diagnostic_from_raw(&raw.diagnostic);
+        }
     }
 }
 
@@ -449,6 +469,25 @@ struct InitOptions {
     file_extension: Option<String>,
     syntax: Option<String>,
     log_level: Option<String>,
+    diagnostic: Option<RawDiagnostic>,
+}
+
+#[derive(Debug, Deserialize)]
+#[serde(rename_all = "snake_case")]
+struct RawDiagnostic {
+    #[serde(default)]
+    link_severity: Option<String>,
+}
+
+/// Build a [`DiagnosticConfig`] from the raw client payload, falling back
+/// to defaults for absent or unrecognised values.
+fn diagnostic_from_raw(raw: &Option<RawDiagnostic>) -> DiagnosticConfig {
+    let link_severity = raw
+        .as_ref()
+        .and_then(|d| d.link_severity.as_deref())
+        .and_then(LinkSeverity::parse)
+        .unwrap_or_default();
+    DiagnosticConfig { link_severity }
 }
 
 #[derive(Debug, Deserialize)]