chore(CI): use custom GitHub app for generation PRs so we can run tests

[niklashigi]

Note

Switches SDK generation workflow to authenticate via a GitHub App and use its token for PRs that can trigger downstream workflows.

• Adds get-token job using actions/create-github-app-token@v1 and exposes token via outputs
• Updates generate job to depend on get-token and use the app token as github_access_token
• Sets permissions: {} and cleans up workflow on: block

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