fix the RPC tests

[xianshijing-lk]

Problem:
When a local track is created, its sid() returns "TR_unknown"

• The real track SID is assigned by the server during publishing and stored in the publication, not the track itself
• The test uses track->sid() which returns "TR_unknown", causing "track not found" error

The Fix:
The test should use track->publication()->sid() instead of track->sid() when unpublishing, like the other working code does