Set unavailable resources in AI Gateway request when MR feature flag is enabled

What does this MR do and why?

When the ai_merge_request_reader_for_chat feature flag is enabled, we send the w[Pipelines Vulnerabilities] as a parameter to the AI Gateway (AIGW) so that it does not include Merge Requests in the list of unavailable resources.

You can see how this list is used in the AIGW here

MR acceptance checklist

Screenshots or screen recordings

How to set up and validate locally

1. Enable v2 feature flag

Feature.enable(:v2_chat_agent_integration)

2. Ask Duo Chat what the unavailable resources are

It should reply with Merge Requests, Pipelines, Vulnerabilities

3. Clear chat with /clear

4. Enable MR support feature flag

Feature.enable(:ai_merge_request_reader_for_chat)

5. Ask Duo Chat what the unavailable resources are

It should reply with Pipelines, Vulnerabilities

Related to #464587 (closed)