In short:
AudienceNetwork (S2S): include ext.security_app_id in OpenRTB requests.
Fixes a production-only 400 ("Error validating authentication token") when test=0,
while test=1 works. Adds field wiring in AudienceNetworkExt and passes it through
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Summary

This PR fixes a production-only failure in the Meta Audience Network (AN) S2S bidding integration within Prebid Server Java.
In production (test=0), AN requires ext.security_app_id alongside ext.authentication_id for request authentication. The field was not present in outgoing requests, leading to 400 responses (x-fb-an-errors: Error validating authentication token).
Adding ext.security_app_id resolves the issue. Test mode (test=1) remained unaffected, which masked the missing field during integration testing.

Problem Statement
# Observed:
• test=1 requests succeed end-to-end.
• test=0 requests fail with HTTP 400, headers include x-fb-an-errors=Error validating authentication token, often with empty body and standard Meta edge headers.
# Impact: Audience Network bidding seat is effectively disabled in production traffic, reducing competition and potential yield.