chore(deps): update dependency @types/koa to v2.15.2

[renovate]

This PR contains the following updates:

Package	Change	Age	Confidence
@types/koa (source)	2.13.5 → 2.15.2

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[renovate]

⚠️ Artifact update problem

Renovate failed to update an artifact related to this branch. You probably do not want to merge this PR as-is.

♻ Renovate will retry this branch, including artifacts, only when one of the following happens:

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The artifact failure details are included below:

File name: site/study/2026/experiments/environments/1618/yarn.lock

error This project's package.json defines "packageManager": "yarn@4.18.0". However the current global version of Yarn is 1.22.22.

Presence of the "packageManager" field indicates that the project is meant to be used with Corepack, a tool included by default with all official Node.js distributions starting from 16.9 and 14.19.
Corepack must currently be enabled by running corepack enable in your terminal. For more information, check out https://yarnpkg.com/corepack.