fix(lsp): retrigger diagnostics request on server cancellation (#31345) (#31427)

Co-authored-by: Jesse <github@jessebakker.com>
(cherry picked from commit 29c72cdf4a)

runtime/lua/vim/lsp/protocol.lua

@@ -166,6 +166,7 @@ local constants = {
     -- Defined by the protocol.
     RequestCancelled = -32800,
     ContentModified = -32801,
+    ServerCancelled = -32802,
   },
 
   -- Describes the content type that a client supports in various