Document AI Gateway mTLS for upstream model endpoints

Manoj M J [OOO, back on 24th]requested to merge
mmj-aigw-upstream-mtls-docs into master

What does this MR do and why?

Documents the new AIGW_MTLS__* environment variables that make the GitLab AI Gateway present a client certificate (mutual TLS) to upstream model endpoints that require one — the equivalent of curl --cert.

Adds a "Connect to a model endpoint that requires mutual TLS (mTLS)" section to doc/install/install_ai_gateway.md, right after the existing self-signed-certificate section. The two are complementary: the existing section covers trusting the upstream server certificate; the new section covers presenting the AI Gateway's own client certificate.

Dependency

This documents functionality added in the AI Gateway MR: gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!6084 (merged)

Kept as a Draft until that MR merges and ships in an AI Gateway image. The Introduced in GitLab 19.2 version note should be confirmed against the actual AI Gateway release once known.

Author checklist

  • Documentation-only change (doc/install/install_ai_gateway.md).
  • Vale reported 0 errors/warnings/suggestions locally.

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