fix: multiplied code exchange requests in strict mode

[Ignatella]

In current implementation with:

• React: 18.2.0 strict mode
• Identity Provider: IdentityServer4
• Flow: authorization code + pkce

We experience multiplied authorization code exchange requests as a result of not handling simulation of component immediate unmounting and remounting in strict mode.

isMountedRef cannot be used as it will be updated during next useEffect:

Fix: local guard variable is introduced:

[Critteros]

Also facing the same issue

[codecov]

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[simenandre]

Thank you so much for this!

It would be great to see a unit test on this at some point.