Add submission list and detail API tests

[Copilot]

Integration tests for submissions index (/contests/{slug}/submissions), all (/contests/{slug}/submissions/all), and show (/contests/{slug}/submissions/{id}) endpoints covering auth, permissions, and visibility rules.

Tests (29 total)

• index (8): unauthorized access, own-submissions-only filtering, pagination, not-found, response shape
• all (9): ended vs running contest visibility, forbidden for anonymous/regular users on running contests, admin/writer access during running contests
• show (12): owner/anonymous/admin/writer access to public/private submissions, contest-slug mismatch, running-contest restrictions, response detail fields

Test infrastructure

• Fixtures for users (admin/writer/member roles), contests (ended/running), problems, submissions (public/private), testcases, registrations
• GCS client stubbed via define_singleton_method in setup/teardown
• Session middleware added to test env for DeviseTokenAuth bypass_sign_in compatibility

Pre-existing fixes required to unblock test execution

• enum syntax updated in Contest and TestcaseSet models (Rails 8 removed the hash-style enum field: {} API)
• GCS client initialization guarded with unless Rails.env.test? to prevent eager-load failures without credentials

# Rails 8 enum syntax fix
-  enum standings_mode: { atcoder: 1, icpc: 2 }
+  enum :standings_mode, { atcoder: 1, icpc: 2 }

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[Phase 1] 提出一覧および詳細 API のテスト作成</issue_title>
<issue_description>- 提出一覧 (/contests/{slug}/submissions) にテストを作成する

• 提出詳細 (/contests/{slug}/submissions/{id})

• 正常系・異常系とも作成

• 権限については慎重に
  -</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [Phase 1] 提出一覧および詳細 API のテスト作成 #103

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.