fix(tools): drop empty required entry from list_task_credentials schema

list_task_credentials takes no parameters but built its JSON Schema with
obj(map[string]any{}, ""), emitting {"type":"object","required":[""],
"properties":{}}. The required array contains an empty string, which strict
function-call validators reject. Observed with DeepSeek through an
OpenAI-compatible gateway (format "openai"):

  openai: status 400 invalid_tool_spec -
  "function.parameters.required must be an array of non-empty strings"

Because the tool is in the worker toolset, every worker run fails before the
first tool call (terminal state model_error); the planner and main agent are
unaffected since they don't load it. Anthropic/OpenAI accept the malformed
schema silently, so it only surfaces on stricter providers.

Pass no required args so obj omits the required key entirely:
{"type":"object","properties":{}}.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

agent/tools_creds.go

@@ -64,7 +64,7 @@ func (t *ToolSet) listTaskCredentials() actool.CoreTool {
 	return writeTool(
 		"list_task_credentials",
 		"List this task's test accounts: user-declared accounts (e.g. program-provided aliases; a user-declared entry with an empty password means: register an account on that domain with a strong password, then record it via save_task_credential) plus accounts recorded by earlier runs. Check this BEFORE registering a new account, and use matching entries for authenticated testing.",
-		obj(map[string]any{}, ""),
+		obj(map[string]any{}),
 		func(_ context.Context, _ json.RawMessage) (actool.Result, error) {
 			if t.ts == nil {
 				return actool.Errorf("list_task_credentials is not enabled: task store is not initialized"), nil