2762: Prevent individual external PR failures from blocking IssueBot processing

[zhaosongzs]

IssueBot previously processed all PRs associated with an updated issue through a single stream. If fetching any one external PR failed, the exception terminated the whole stream. Therefore, valid PRs were not processed and no CheckWorkItems were created for them. The periodic batch also did not complete normally.

We should process each associated PR independently instead. When a PR query fails, log the failure and schedule the issue for retry in a later polling round. Continue creating CheckWorkItems for the remaining PRs in the current round, so a transient or broken external PR cannot block unrelated valid PRs.

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Progress

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Issue

• SKARA-2762: Prevent individual external PR failures from blocking IssueBot processing (Bug - P3)

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Update a local copy of the PR:
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$ git pr checkout 1781

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Webrevs

• 00: Full (daae3c3f)