refactor(lsp): remove implicit rpc error tostring #38707

Problem:
LSP error responses implicitly rely on a custom `__tostring` function
(`vim.lsp.rpc.format_rpc_error`) for formatting. This causes errors that are not
created via `vim.lsp.rpc.error` to behave inconsistently with those that are.

Furthermore, we usually use `log.error` to print these errors, which uses
`vim.inspect` under the hood, so the custom `__tostring`  provides little
benefit.

This increases the difficulty of refactoring the code, as it tightly couples RPC
error handling with the LSP.

Solution:
Convert every potential `__tostring` call to an explicit one. Since we don't
describe this behavior in the documentation, this should not be a breaking
change.

runtime/lua/vim/lsp/client.lua

@@ -578,7 +578,9 @@ function Client:initialize()
   local rpc = self.rpc
 
   rpc.request('initialize', init_params, function(init_err, result)
-    assert(not init_err, tostring(init_err))
+    if init_err then
+      error(lsp.rpc.format_rpc_error(init_err))
+    end
     assert(result, 'server sent empty result')
     rpc.notify('initialized', vim.empty_dict())
     self.initialized = true