E2E: Add AiGatewayNoSeatAssigned Tests

What does this MR do and why?

Relates to https://gitlab.com/gitlab-org/quality/quality-engineering/team-tasks/-/issues/2504

This MR adds a new orchestrated ai-gateway-no-seat-assigned job, along with new E2E tests within the following files:

• ee/api/3_create/code_suggestions_spec.rb | Test case: https://gitlab.com/gitlab-org/gitlab/-/quality/test_cases/451487
• ee/browser_ui/3_create/web_ide/code_suggestions_in_web_ide_spec.rb | Test case: https://gitlab.com/gitlab-org/gitlab/-/quality/test_cases/451486

These tests verify that a user is unable to access code suggestions via the API and the Web IDE when they do not have a Duo Pro seat assigned to them.

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