test(session-title): mark the assertNever guard arm v8-ignored

The default arm of copySessionTitleSource is a closed-union exhaustiveness
guard (compile-time protection for the next kind); it is unreachable at
runtime, so the per-file coverage gate ignores it like every other
assertNever arm.

packages/session-title/session-title/src/index.ts

@@ -200,6 +200,7 @@ function copySessionTitleSource(source: SessionTitleSource): SessionTitleSource
       ...(source.model === undefined ? {} : { model: { ...source.model } }),
     }
     case 'user': return { kind: 'user' }
+    /* v8 ignore next -- closed-union exhaustiveness guard */
     default: return assertNever(source, 'SessionTitleSource')
   }
 }