fix(lsp): ensure bufnr is passed for buffer-local requests (#36325)

Problem:
Some LSP method handlers were making requests without specifying a
bufnr, defaulting to 0 (current). This works in most cases but
fails when client attaches to background buffers, causing
assertions in handlers to fail.

Solution:
Ensure bufnr is passed to Client.request for buffer-local methods.

runtime/lua/vim/lsp/diagnostic.lua

@@ -264,7 +264,7 @@ function M.on_diagnostic(error, result, ctx)
     if error.data == nil or error.data.retriggerRequest ~= false then
       local client = assert(lsp.get_client_by_id(ctx.client_id))
       ---@diagnostic disable-next-line: param-type-mismatch
-      client:request(ctx.method, ctx.params)
+      client:request(ctx.method, ctx.params, nil, ctx.bufnr)
     end
     return
   end