updateAssetParameterMacros

Merge macros into the asset-parameter macros of the active session. Each name replaces the previous value for that name; other names stay. A null value removes the customer macro so the built-in $OPTIVIEW_* value (if any) applies again. When a GAM stream is active the effective ad tag parameters are resolved again and pushed to IMA; VAST tag URLs use the new values on the next ad request. No-op (with a warning) without an active session.