fix: address PR review feedback on editor/getDiagnostics

- send_response guards against non-running server process
- add send_error_response for spec-compliant JSON-RPC error replies
- unknown methods return nil -> -32601 Method Not Found error response
- pcall-wrap error-path sends to prevent crash on shutdown race
- rename test variable count -> diagnostics for clarity

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: ssjoleary

lua/eca/editor.lua

@@ -39,7 +39,7 @@ function M.handle_request(message)
   if message.method == "editor/getDiagnostics" then
     return get_diagnostics(message.params or {})
   end
-  return {}
+  return nil
 end
 
 return M

lua/eca/server.lua

@@ -267,7 +267,11 @@ function M:handle_message(message)
         local ok, result = pcall(self.on_request, self, message)
         if not ok then
           Logger.error("on_request error: " .. tostring(result))
-          self:send_response(id, {})
+          pcall(self.send_error_response, self, id, -32603, "Internal error")
+          return
+        end
+        if result == nil then
+          pcall(self.send_error_response, self, id, -32601, "Method not found")
           return
         end
         local ok2, err = pcall(self.send_response, self, id, result)
@@ -286,12 +290,30 @@ end
 ---@param id integer|string
 ---@param result table
 function M:send_response(id, result)
+  if not self:is_running() then
+    Logger.error("send_response: server not running, dropping response for id=" .. tostring(id))
+    return
+  end
   local message = { jsonrpc = "2.0", id = id, result = result }
   local json = vim.json.encode(message)
   local content = string.format("Content-Length: %d\r\n\r\n%s", #json, json)
   self.process:write(content)
 end
 
+---@param id integer|string
+---@param code integer JSON-RPC error code
+---@param msg string
+function M:send_error_response(id, code, msg)
+  if not self:is_running() then
+    Logger.error("send_error_response: server not running, dropping error for id=" .. tostring(id))
+    return
+  end
+  local message = { jsonrpc = "2.0", id = id, error = { code = code, message = msg } }
+  local json = vim.json.encode(message)
+  local content = string.format("Content-Length: %d\r\n\r\n%s", #json, json)
+  self.process:write(content)
+end
+
 ---@return integer
 function M:get_next_id()
   self.next_id = self.next_id + 1

tests/test_editor_diagnostics.lua

@@ -14,13 +14,13 @@ local T = MiniTest.new_set({
 T["editor"] = MiniTest.new_set()
 
 T["editor"]["returns empty diagnostics when buffer not found"] = function()
-  local count = child.lua_get([[
+  local diagnostics = child.lua_get([[
     require("eca.editor").handle_request({
       method = "editor/getDiagnostics",
       params = { uri = "file:///nonexistent/file.lua" },
     }).diagnostics
   ]])
-  eq(#count, 0)
+  eq(#diagnostics, 0)
 end
 
 T["editor"]["maps severity levels correctly"] = function()
@@ -113,11 +113,11 @@ T["editor"]["uri field is present on each diagnostic"] = function()
   eq(child.lua_get("_G.uri_result[2]"), "file:///tmp/eca_test_uri.lua")
 end
 
-T["editor"]["returns empty table for unknown method"] = function()
-  local count = child.lua_get([[
-    vim.tbl_count(require("eca.editor").handle_request({ method = "editor/unknown", params = {} }))
+T["editor"]["returns nil for unknown method"] = function()
+  local is_nil = child.lua_get([[
+    require("eca.editor").handle_request({ method = "editor/unknown", params = {} }) == nil
   ]])
-  eq(count, 0)
+  eq(is_nil, true)
 end
 
 return T