Add 'third_party_agent_token_created' audit event

What does this MR do and why?

Add 'third_party_agent_token_created' audit event:

• external agent name (Amazon Q, Claude, etc.)
• token scope and expiration in audit event
• associated workflow/session

References

• https://gitlab.com/gitlab-org/gitlab/-/work_items/593031

Screenshots or screen recordings

Before	After

How to set up and validate locally

In rails console:

user = User.first # or root user
project = Project.find(<id>) # gitlab duo enabled project

result = Ai::ThirdPartyAgents::TokenService.new(
  current_user: user,
  project: project,
  agent_name: 'Amazon Q Developer Agent',
  workflow_id: 123
).direct_access_token

AuditEvent.last

Example:

 id: 20124,
 author_id: 1,
 entity_id: 1,
 entity_type: "User",
 details:
  {:token_expires_at=>1773783251, :token_scopes=>["complete_code", "ai_gateway_model_provider_proxy"], :agent_name=>"Amazon Q Developer Agent", :workflow_id=>123, :project_id=>30, :namespace_id=>1000000, :event_name=>"third_party_agent_token_created", :author_name=>"Administrator", :author_class=>"User", :target_id=>1, :target_type=>"User", :target_details=>"Administrator", :custom_message=>"Generated third-party agent access token", :ip_address=>"172.16.123.1", :entity_path=>"root"},
 ip_address: #<IPAddr: IPv4:172.16.123.1/255.255.255.255>,
 author_name: "Administrator",
 entity_path: "root",
 target_details: "Administrator",
 created_at: "2026-03-17 20:34:11.312341000 +0000",
 target_type: "User",
 target_id: 1>

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.