fix(app-check): Prevent redundant exchangeToken calls in debug mode

[nicole0707]

This PR addresses a race condition in getToken() where multiple concurrent calls could each pass the !state.exchangeTokenPromise check before any of them had set the promise, resulting in redundant exchangeToken() network requests.

Changes

• Moved the await getDebugToken() call before the if (!state.exchangeTokenPromise) check to ensure token preparation occurs before promise coordination logic.
• Assigned state.exchangeTokenPromise immediately, ensuring concurrent calls share the same in-flight promise.
• Add unit tests

Notes

Resolved #9184

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[hsubox76]

Great PR, thanks! There's one nit (typo) and you need to run yarn format and I think it should be good.

[hsubox76]

nit: typo -appCheckServie

[nicole0707]

Thanks @hsubox76 , I've updated the branch.

[hsubox76]

Thanks again, this should go out in the next release, which is planned for Aug 7.

[nicole0707]

Thanks again, this should go out in the next release, which is planned for Aug 7.

Thanks @hsubox76 🚀