fix(slack): use SLACK_USER_TOKEN when connecting to Slack (#28103)

* fix(slack): use SLACK_USER_TOKEN when connecting to Slack (closes #26480)

* test(slack): fix account fixture typing for user token source

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Co-authored-by: Tak Hoffman <781889+Takhoffman@users.noreply.github.com>

src/slack/token.ts

@@ -10,3 +10,7 @@ export function resolveSlackBotToken(raw?: string): string | undefined {
 export function resolveSlackAppToken(raw?: string): string | undefined {
   return normalizeSlackToken(raw);
 }
+
+export function resolveSlackUserToken(raw?: string): string | undefined {
+  return normalizeSlackToken(raw);
+}