feat(wallet): wire real sendRaw() — Phase 2 complete

- RustTimeline.sendRaw() now calls inner.sendRaw() via custom SDK .aar
- DefaultPaymentEventSender fully implemented: serializes payment data as JSON,
  sends co.sulkta.payment.request and co.sulkta.payment.status event types
- matrix-rust-sdk.aar built from sulkta/send-raw-v1 fork with UniFFI binding
- Removes UnsupportedOperationException stub — payments now actually send

libraries/matrix/impl/src/main/kotlin/io/element/android/libraries/matrix/impl/timeline/RustTimeline.kt

@@ -280,15 +280,20 @@ class RustTimeline(
     }
 
     /**
-     * Send a raw/custom event. Currently not supported by the Rust SDK bindings.
-     * The SDK Timeline does not expose sendRaw - custom events must use message markers for now.
+     * Send a raw/custom event to the room.
+     *
+     * @param eventType The event type (e.g., "co.sulkta.payment.request")
+     * @param content The JSON content of the event
+     * @return Result indicating success or failure
      */
     override suspend fun sendRaw(
         eventType: String,
         content: String,
-    ): Result<Unit> {
-        // The Rust SDK Timeline interface does not expose sendRaw yet.
-        return Result.failure(UnsupportedOperationException("sendRaw not yet supported by Matrix Rust SDK bindings"))
+    ): Result<Unit> = withContext(dispatcher) {
+        runCatchingExceptions {
+            inner.sendRaw(eventType, content)
+            Unit
+        }
     }
 
     override suspend fun redactEvent(eventOrTransactionId: EventOrTransactionId, reason: String?): Result<Unit> = withContext(dispatcher) {