feat(inbox): include linear id in data source creation

[jonathanlab]

We were not passing our linear_integration_id into the external data source create call, causing it to fail and Linear integrations to not work.

Closes #1495
Closes #2005
https://discord.com/channels/1465397904901673123/1465397906977849612/1501995293884022795
https://posthog.slack.com/archives/C09G8Q32R6F/p1778194129515839

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Created with PostHog Code

[greptile-apps]

Prompt To Fix All With AI

Fix the following 1 code review issue. Work through them one at a time, proposing concise fixes.

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### Issue 1 of 1
apps/code/src/renderer/features/inbox/components/DataSourceSetup.tsx:289-291
The type assertion here is unsound — the `find` callback's element is typed as `{ kind: string }`, which is then cast to the unrelated `{ id: number | string } | undefined`. If the API response shape ever changes, TypeScript won't catch the breakage. Better to widen the callback type to include `id` so that the property access is verified rather than assumed.

```suggestion
          const linearIntegration = integrations.find(
            (i: { kind: string; id: number | string }) => i.kind === "linear",
          );
```

_{Reviews (1): Last reviewed commit: "feat(inbox): include linear id in data s..." | Re-trigger Greptile}