fix(CI): update GitHub App token secrets for SDK generation

[niklashigi]

Note

Low Risk
Small CI-only change that just swaps which GitHub App credentials are used to mint a token; risk is limited to potentially breaking the SDK generation workflow if the new secrets are misconfigured.

Overview
Switches the SDK Generation GitHub Actions workflow to generate its GitHub App token from SDK_GENERATOR_GITHUB_APP_ID / SDK_GENERATOR_GITHUB_APP_PRIVATE_KEY instead of the previous generic app secrets.

This only affects the authentication used by the get-token job before invoking the downstream Speakeasy SDK generation workflow.

^{Written by Cursor Bugbot for commit f59664e. This will update automatically on new commits. Configure here.}