google_ads: new campaign source builds a query that grows until it fails

## Problem

1. The source excludes campaigns it has already seen by adding one `campaign.id != X` clause per campaign, and the saved list is never trimmed. The query grows on every run until the API rejects it. After that every run fails and no events are emitted.
2. The filter duplicates the platform dedupe the source already uses, so it is not needed.
3. Sources poll every 15 minutes by default, which is 96 runs a day. New campaigns appear rarely, so almost every run returns nothing.

## What's needed

- Deduplication that does not grow the query.
- A default interval closer to how often the event actually happens.

## Notes

- Failed requests that return a Google Ads failure count against the daily quota: https://developers.google.com/google-ads/api/docs/best-practices/quotas
- GAQL grammar and query constraints: https://developers.google.com/google-ads/api/docs/query/grammar
- Context: quota exhaustion reported in #19945, #21509, #21554, #21782, #21926.