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/doc/lsp.txt

@@ -2264,7 +2264,10 @@ get({filter})                                       *vim.lsp.inlay_hint.get()*
 
         local client = vim.lsp.get_client_by_id(hint.client_id)
         local resp = client:request_sync('inlayHint/resolve', hint.inlay_hint, 100, 0)
-        local resolved_hint = assert(resp and resp.result, resp.err)
+        local resolved_hint = assert(
+          resp and resp.result,
+          resp and resp.err and vim.lsp.rpc.format_rpc_error(resp.err) or 'request failed'
+        )
         vim.lsp.util.apply_text_edits(resolved_hint.textEdits, 0, client.encoding)
 
         location = resolved_hint.label[1].location

runtime/lua/vim/lsp/buf.lua

@@ -1460,7 +1460,8 @@ function M.selection_range(direction, timeout_ms)
   timeout_ms = timeout_ms or 1000
   local result, err = lsp.buf_request_sync(0, method, params, timeout_ms)
   if err then
-    lsp.log.error('selectionRange request failed: ' .. err)
+    local err_message = type(err) == 'table' and lsp.rpc.format_rpc_error(err) or err
+    lsp.log.error('selectionRange request failed: ' .. err_message)
     return
   end
   if not result or not result[client.id] or not result[client.id].result then

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

runtime/lua/vim/lsp/inlay_hint.lua

@@ -152,7 +152,10 @@ end
 ---
 --- local client = vim.lsp.get_client_by_id(hint.client_id)
 --- local resp = client:request_sync('inlayHint/resolve', hint.inlay_hint, 100, 0)
---- local resolved_hint = assert(resp and resp.result, resp.err)
+--- local resolved_hint = assert(
+---   resp and resp.result,
+---   resp and resp.err and vim.lsp.rpc.format_rpc_error(resp.err) or 'request failed'
+--- )
 --- vim.lsp.util.apply_text_edits(resolved_hint.textEdits, 0, client.encoding)
 ---
 --- location = resolved_hint.label[1].location

runtime/lua/vim/lsp/rpc.lua

@@ -137,13 +137,11 @@ function M.rpc_response_error(code, message, data)
   -- TODO should this error or just pick a sane error (like InternalError)?
   ---@type string
   local code_name = assert(protocol.ErrorCodes[code], 'Invalid RPC error code')
-  return setmetatable({
+  return {
     code = code,
     message = message or code_name,
     data = data,
-  }, {
-    __tostring = M.format_rpc_error,
-  })
+  }
 end
 
 --- Dispatchers for LSP message types.
@@ -540,9 +538,6 @@ function Client:handle_body(body)
     if callback then
       self.message_callbacks[result_id] = nil
       validate('callback', callback, 'function')
-      if decoded.error then
-        setmetatable(decoded.error, { __tostring = M.format_rpc_error })
-      end
       self:try_call(
         M.client_errors.SERVER_RESULT_CALLBACK_ERROR,
         callback,