also cast doc_id to str

[seanstory]

Closes https://github.com/elastic/search-team/issues/8956

We were ensuring that the ingested doc[id] was a string, but at the time we computed:

                if doc_id in existing_ids:

doc_id could be numeric, but existing_ids were guaranteed to be strings.

Checklists

Pre-Review Checklist

• this PR does NOT contain credentials of any kind, such as API keys or username/passwords (double check config.yml.example)
• this PR has a meaningful title
• this PR links to all relevant github issues that it fixes or partially addresses
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Release Note

Fixes a bug where full syncs may delete documents they just ingested if the document ID when fetched from the 3rd party was numeric.