Add usage credit check to web template

Tetiana Chuprynarequested to merge
brytannia-chat-credits-please-dont-break-pipeline into master

What does this MR do and why?

Connects !214525 (merged) with Backend service and fixes failed tests

This is last piece required for https://gitlab.com/gitlab-org/gitlab/-/issues/581064+s

The history if this MR: Initially integration between FE and BE was done in !214525 (merged) but due to test failures we decided to extract backend part to this MR.

Related slack thread: https://gitlab.slack.com/archives/C06LWENL58F/p1765380617717689

NOTE: this MR is blocker for Duo Agent Platform GA, please help it to land successfully in 18.7

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References https://gitlab.com/gitlab-org/gitlab/-/work_items/583257+

Edited by Denys Mishunov

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