fix: implement cursor pagination with JSON body (body_json)

[sergibeltran]

Problem

When paginating a POST API with cursor pagination where the nextPageToken cursor must be sent inside the JSON request body, Infinity always returned the first page. The body_json pagination param type was declared in the models but never implemented — the cursor was appended as a query parameter instead of being injected into the body, so every request was identical.

Changes

• Implement the body_json pagination param type in ApplyPaginationItemToQuery: the cursor key/value is merged into the request JSON body (guarded so a non-JSON body is left untouched rather than overwritten).
• Expose the "Body JSON" option in the pagination param type dropdown in the query editor (it was commented out).
• Handle the final page gracefully: many cursor APIs omit the cursor field on the last page, which previously caused error while extracting the cursor value and failed the whole query. This is now treated as "no more pages".

Testing

• Added unit tests for body_json injection (empty body, existing body, overwrite existing key, non-JSON body left untouched).
• go test ./..., go vet, yarn typecheck, yarn lint, yarn test:ci and yarn spellcheck all pass.
• Verified end-to-end against a real POST API (Jira /rest/api/3/search/jql) paginating 3500+ records via nextPageToken in the JSON body.

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