fix: add BitcoinNative branch to exhaustive Chain.Asset.Type whens

CI caught onChainAssetId's when as non-exhaustive; a codebase-wide sweep
for every other exhaustive when on Chain.Asset.Type found one more
(existentialDepositError in Common.kt) that would have failed the same way.

runtime/src/main/java/io/novafoundation/nova/runtime/ext/ChainExt.kt

@@ -637,6 +637,7 @@ val Chain.Asset.onChainAssetId: String?
         is Type.EvmNative -> null
         is Type.Trc20 -> this.type.contractAddress
         Type.TronNative -> null
+        Type.BitcoinNative -> null
         Type.Unsupported -> error("Unsupported assetId type: ${this.type::class.simpleName}")
     }