RTBHouse: Fix unintended site object creation for app requests

[pjaworski-rtbh]

🔧 Type of changes

• bid adapter update
• bugfix

✨ What's the context?

The RTBHouse bidder was unconditionally constructing a site object on every outgoing request while attaching the publisher ID. For app-only requests (no site in the incoming Prebid Server request), this resulted in a synthetic site object being added to the outbound bid request, even though only an app object was present.

📋 Description

Fixes publisher ID propagation so it no longer fabricates a site object:

1. modifySite now only modifies an existing site and returns null untouched — no synthetic site is created when the request has none.
2. New modifyApp propagates the publisher ID to app.publisher.ext.prebid.publisherId for app traffic, mirroring the existing site.publisher behaviour.
3. Extracted a shared modifyPublisher helper used by both site and app.

🧪 Testing results

Tests run: 26, Failures: 0, Errors: 0, Skipped: 0
All RTBHouse bidder tests pass, including new coverage:

• makeHttpRequestsShouldNotCreateSiteWhenBidRequestHasAppOnly — verifies no site is added for app-only requests
• makeHttpRequestsShouldAddPublisherToAppWhenBidRequestHasAppOnly — verifies publisher ID is propagated to app.publisher
• Existing site-path and backward-compatibility tests retained

Other information

Please reach us at inventory_support@rtbhouse.com with piotr.jaworski@rtbhouse.com and cc leandro.otani@rtbhouse.com.