fix(nms): Fix fetching of subscriber config for paginated subscribers

[voisey]

Summary

Closes #13506 by implementing the fallback mechanism that is present in SubscriberDetail to SubscriberDetailConfig

Test Plan

• Reproduced the bug locally, then verified that with this fix, refreshing the subscriber config page for subscribers not on the first page doesn't lead to an error
• CI

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[github-actions]

dp-workflow

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Results for commit d5d2bab.

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[github-actions]

nms-workflow

0 tests   0 ✔️  0s ⏱️
0 suites  0 💤
0 files    0 ❌

Results for commit d5d2bab.