test(httpapi): add GetDockerCLISource to fakeRuntimeStats for schema v5

The RuntimeStats interface gained GetDockerCLISource() (MCP-2745); the httpapi
telemetry payload test's fake stub must implement it or the package fails to
typecheck under the full-repo CI lint.

Related #696

Author: Dumbris

internal/httpapi/telemetry_payload_test.go

@@ -36,6 +36,7 @@ func (fakeRuntimeStats) GetRoutingMode() string            { return "retrieve_to
 func (fakeRuntimeStats) IsQuarantineEnabled() bool         { return true }
 func (fakeRuntimeStats) IsDockerAvailable() bool           { return false }
 func (fakeRuntimeStats) GetDockerIsolatedServerCount() int { return 0 }
+func (fakeRuntimeStats) GetDockerCLISource() string        { return "absent" }
 
 func TestHandleGetTelemetryPayload_OK(t *testing.T) {
 	logger := zap.NewNop().Sugar()