feat(mwa): add features[] to CapabilitiesResult for MWA 2.0 completeness

PR #269 shipped GetCapabilities() but its CapabilitiesResult is missing
the features[] array defined in the MWA 2.0 spec. Real wallets populate
it: tested on Solana Seeker with Phantom on mainnet, get_capabilities
returns features=[supports_sign_and_send_transactions]. Without this
field, the SDK silently drops that data on deserialization.

Non-breaking: adds one nullable property on an existing response model.
Existing consumers unaffected.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: mstevens843

Runtime/codebase/SolanaMobileStack/JsonRpcClient/Responses/CapabilitiesResult.cs

@@ -17,4 +17,7 @@ public class CapabilitiesResult
 
     [JsonProperty("supported_transaction_versions")]
     public string[] SupportedTransactionVersions { get; set; }
+
+    [JsonProperty("features", NullValueHandling = NullValueHandling.Ignore)]
+    public string[] Features { get; set; }
 }