fix(auth): associate unAuth identityId to newly authenticated user's identityId

[Samaritan1011001]

Description of changes

Currently, we create and maintain separate identityIds for unAuth (guest) user and newly authenticated user. With this change, we will "upgrade" the unAuth user's identityId to the new authenticated user.

Issue #, if available

#14197

Description of how you validated changes

Action	Expected identityId
fetchAuthSession BEFORE signIn	ID1
fetchAuthSession AFTER signUp NEW user U1	ID1
fetchAuthSession AFTER signIn NEW user U1	ID1 (linked login)
fetchAuthSession AFTER signOut	ID2
fetchAuthSession AFTER signIn EXISTING user U1	ID1
fetchAuthSession AFTER 2nd singOut	ID3
fetchAuthSession AFTER signUp NEW user U2	ID3
fetchAuthSession AFTER signIn NEW user U2	ID3 (linked login)
fetchAuthSession AFTER signIn EXISTING user U1	ID1
fetchAuthSession BEFORE signIn & AFTER signOut	ID4 → ID5

Checklist

• PR description included
• yarn test passes
• Unit Tests are changed or added
• Relevant documentation is changed or added (and PR referenced)

Checklist for repo maintainers

• Verify E2E tests for existing workflows are working as expected or add E2E tests for newly added workflows
• New source file paths included in this PR have been added to CODEOWNERS, if appropriate

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[thesnups]

Thank you for working on this!