[source-hubspot] `leads` missing from engagement stream associations

> 🤖 *This issue was generated by an AI Agent.*

## What's missing

The five engagement streams (`engagements_calls`, `engagements_emails`, `engagements_meetings`, `engagements_notes`, `engagements_tasks`) do not request `leads` associations, so lead-to-engagement relationships are unavailable.

## Verified on `master`

All five engagement stream definitions in `airbyte-integrations/connectors/source-hubspot/manifest.yaml` list the same four associations:

```yaml
associations:
- companies
- contacts
- deals
- tickets
```

The configurable association-stream framework added in #71259 is a separate mechanism (standalone association streams) and does not cover the inline associations emitted on engagement records.

## Origin

Closed as stale: #74030 (@hlepouse-vasco), reviewed 2026-04-17 with no author response.

Filed to keep a record of the gap. The PR was closed for inactivity, not on merit — credit to the original contributor if this is picked up.

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**Internal Tracking:** https://github.com/airbytehq/oncall/issues/13176