fix(lsp): retrigger diagnostics request on server cancellation (#31345)

Co-authored-by: Jesse <github@jessebakker.com>

runtime/lua/vim/lsp/protocol.lua

@@ -174,6 +174,7 @@ local constants = {
     -- Defined by the protocol.
     RequestCancelled = -32800,
     ContentModified = -32801,
+    ServerCancelled = -32802,
   },
 
   -- Describes the content type that a client supports in various