Introduce an optional test job for AI related rspecs

What does this MR do and why?

This MR introduces an optional test job that runs the rspecs tagged with real_ai_request rspec metadata tag.

• The rspecs tagged with real_ai_request test the code that makes actual external requests to AI vendors (Anthropic, OpenAI, Google Vertex, etc.)
  • The only rspec run in the job for now is: ee/spec/lib/gitlab/llm/chain/agents/zero_shot/executor_real_requests_spec.rb.
• The new job is strictly EE-only, allowed to fail and optional.
• To run the job, at least two key pairs are needed. They've been added as masked variables (OPENAI_KEY and ANTHROPIC_API_KEY) to GitLab project settings.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #427250 (closed)