[source-linkedin-ads] Add a videos stream to expose video duration and media metadata

### Topic

[source-linkedin-ads] Add a videos stream to expose video duration and media metadata

### Relevant information

The LinkedIn Ads connector (v5.6.9) has no stream exposing video assets. Marketing teams analyzing video ad performance need the video duration to compute completion rates and compare creatives, but none of the 19 existing streams return it.

The data is available via the LinkedIn Videos API: GET /rest/videos?q=associatedAccount&associatedAccount=urn:li:sponsoredAccount:{id} returns all videos in a sponsored account's media library with duration (ms), thumbnail, aspect ratio and status. The read-only r_ads scope already required by the connector is sufficient.

I have a working implementation (manifest-only, modeled on the existing conversions stream: account partition router, offset pagination, LinkedIn-Version 202601) and will open a PR referencing this issue.

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**Internal Tracking:** https://github.com/airbytehq/oncall/issues/13071