feat(duo ask): self-hosted instance url check

Description

• feat(duo ask): self-hosted instance url check
• Implemented support for self-hosted GitLab instances in ask duo cli command

Related Issues

Resolves https://gitlab.com/gitlab-com/ops-sub-department/aws-gitlab-ai-integration/integration-motion-planning/-/issues/619

How has this been tested?

1. Setup a self-hosted GitLab instance
2. Install GitLab cli tool
3. Configure cli tool via glab auth login using https://gitlab.com/gitlab-community/gitlab-org/cli#configuration instruction
4. Issue glab duo ask reques, ex: glab duo ask list last 10 commit titles

Screenshots (if appropriate):

Error shown to user:

x Error: API is unreachable. error=POST https://gitlab.com/api/v4/ai/llm/git_command: 401 {message: 401 Unauthorized}

After change:

$ glab duo ask list last 10 commit titles

x Error: API is unreachable. error=POST https://gdk.e2e-beta.gitlab-qdev.ai.aws.dev:3443/api/v4/ai/llm/git_command: 401 {message: 401 Unauthorized}

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation
• Chore (Related to CI or Packaging to platforms)
• Test gap