Revert "Merge branch '605887-fix-direct-access-400-on-saas' into 'master'"

What does this MR do and why?

Revert "Merge branch '605887-fix-direct-access-400-on-saas' into 'master'"

This reverts merge request !248253 (merged)

!248253 (merged) let POST /code_suggestions/direct_access succeed on GitLab.com, enabling the Language Server's direct-connection path to AI Gateway for code completions. That path never carries root_namespace_id (no per-request headers, JWT doesn't set it, AI Gateway drops unset params), and CustomersDot requires it for saas claims — so every direct-connection completion request 422'd as "Root namespace can't be blank."

Prod impact: saas/code_completions resolve calls went from ~28/20min to 9,759/20min after !248253 (merged) deployed; 9,734 of those had no root_namespace_id. ~2,339 users affected in one 30-min window.

Why it's safe to revert

Restores the unconditional 400 on /code_suggestions/direct_access, which just forces the Language Server back to its existing fallback: the indirect path through the monolith. Completions keep working, just without the direct-connection speed-up.

Follow-up

Re-land !248253 (merged)'s fix only once direct-access tokens can carry root_namespace_id through to the usage-quota check.

References

Incident #inc-12778-increased-rate-of-usage-billing-blocks-for-an-error-block-reason

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Before	After

How to set up and validate locally

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