test(channelserver): replace time.Sleep with polling loops

Blind sleeps accumulate serially (no t.Parallel anywhere) and inflate
under the race detector's scheduling overhead — contributing to the
~136s channelserver test run time.

Replace ~75 arbitrary sleeps (50ms–1s) across 7 test files with 2s
polling loops that exit as soon as the expected condition holds. Sleeps
that are genuinely intentional (race-condition stress tests, cache
expiry, temporal spacing in timestamp tests, backpressure pacing) are
left untouched.

server/channelserver/handlers_rengoku_integration_test.go

@@ -229,7 +229,6 @@ func TestRengokuData_SaveLoadRoundTrip_AcrossSessions(t *testing.T) {
 
 	// Logout session 1
 	logoutPlayer(session1)
-	time.Sleep(100 * time.Millisecond)
 
 	// === SESSION 2: Load data in new session ===
 	session2 := createTestSessionForServerWithChar(server, charID, "RengokuChar2")
@@ -348,7 +347,6 @@ func TestRengokuData_SkillRegionPreserved(t *testing.T) {
 	handleMsgMhfSaveRengokuData(session1, savePkt)
 	drainAck(t, session1)
 	logoutPlayer(session1)
-	time.Sleep(100 * time.Millisecond)
 
 	// === SESSION 2: Load and verify skill region ===
 	session2 := createTestSessionForServerWithChar(server, charID, "SkillChar")