[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

Start with the LinkedIn Ads connector's existing conversions stream and compare its account partition router, offset pagination, and LinkedIn-Version 202601 settings with the LinkedIn Videos API endpoint described here. Add the videos stream so it exposes duration, thumbnail, aspect ratio, and status, then verify that it reads all videos for the sponsored account.

Escrita pelo modelo de indexação a partir do texto da issue.