feat: Add mempool_get_info method for v1.6 mempool fee information

Add the `mempool.get_info` RPC method to the ElectrumApi trait. This
method was added in protocol v1.6 and provides more detailed mempool
fee information than the deprecated `blockchain.relayfee` method.

The response includes:
- `mempoolminfee`: Dynamic minimum fee rate for tx acceptance
- `minrelaytxfee`: Static operator-configured minimum relay fee
- `incrementalrelayfee`: Minimum fee rate increment for RBF/mempool limiting

Co-Authored-By: Claude Code AI
Signed-off-by: Elias Rohrer <dev@tnull.de>

Author: tnull

src/api.rs

@@ -172,6 +172,10 @@ where
         (**self).server_features()
     }
 
+    fn mempool_get_info(&self) -> Result<MempoolInfoRes, Error> {
+        (**self).mempool_get_info()
+    }
+
     fn ping(&self) -> Result<(), Error> {
         (**self).ping()
     }
@@ -398,6 +402,12 @@ pub trait ElectrumApi {
     /// Returns the capabilities of the server.
     fn server_features(&self) -> Result<ServerFeaturesRes, Error>;
 
+    /// Returns information about the current state of the mempool.
+    ///
+    /// This method was added in protocol v1.6 and replaces `relay_fee` by providing
+    /// `minrelaytxfee` along with additional mempool fee information.
+    fn mempool_get_info(&self) -> Result<MempoolInfoRes, Error>;
+
     /// Pings the server. This method can also be used as a "dummy" call to trigger the processing
     /// of incoming block header or script notifications.
     fn ping(&self) -> Result<(), Error>;
@@ -607,6 +617,10 @@ mod test {
             unreachable!()
         }
 
+        fn mempool_get_info(&self) -> Result<super::MempoolInfoRes, super::Error> {
+            unreachable!()
+        }
+
         fn ping(&self) -> Result<(), super::Error> {
             unreachable!()
         }

src/client.rs

@@ -362,6 +362,11 @@ impl ElectrumApi for Client {
         impl_inner_call!(self, server_features)
     }
 
+    #[inline]
+    fn mempool_get_info(&self) -> Result<MempoolInfoRes, Error> {
+        impl_inner_call!(self, mempool_get_info)
+    }
+
     #[inline]
     fn ping(&self) -> Result<(), Error> {
         impl_inner_call!(self, ping)

src/raw_client.rs

@@ -1179,6 +1179,17 @@ impl<T: Read + Write> ElectrumApi for RawClient<T> {
         Ok(serde_json::from_value(result)?)
     }
 
+    fn mempool_get_info(&self) -> Result<MempoolInfoRes, Error> {
+        let req = Request::new_id(
+            self.last_id.fetch_add(1, Ordering::SeqCst),
+            "mempool.get_info",
+            vec![],
+        );
+        let result = self.call(req)?;
+
+        Ok(serde_json::from_value(result)?)
+    }
+
     fn ping(&self) -> Result<(), Error> {
         let req = Request::new_id(
             self.last_id.fetch_add(1, Ordering::SeqCst),
@@ -1227,6 +1238,16 @@ mod test {
         assert_eq!(resp.pruning, None);
     }
 
+    #[test]
+    fn test_mempool_get_info() {
+        let client = get_test_client();
+
+        let resp = client.mempool_get_info().unwrap();
+        assert!(resp.mempoolminfee >= 0.0);
+        assert!(resp.minrelaytxfee >= 0.0);
+        assert!(resp.incrementalrelayfee >= 0.0);
+    }
+
     #[test]
     #[ignore = "depends on a live server"]
     fn test_batch_response_ordering() {

src/types.rs

@@ -202,6 +202,21 @@ pub struct ServerFeaturesRes {
     pub pruning: Option<i64>,
 }
 
+/// Response to a [`mempool_get_info`](../client/struct.Client.html#method.mempool_get_info) request.
+///
+/// Contains information about the current state of the mempool.
+#[derive(Clone, Debug, Deserialize)]
+pub struct MempoolInfoRes {
+    /// Dynamic minimum fee rate in BTC/kvB for tx to be accepted given current conditions.
+    /// The maximum of `minrelaytxfee` and minimum mempool fee.
+    pub mempoolminfee: f64,
+    /// Static operator-configurable minimum relay fee for transactions, in BTC/kvB.
+    pub minrelaytxfee: f64,
+    /// Static operator-configurable minimum fee rate increment for mempool limiting or
+    /// replacement, in BTC/kvB.
+    pub incrementalrelayfee: f64,
+}
+
 /// Response to a [`server_features`](../client/struct.Client.html#method.server_features) request.
 #[derive(Clone, Debug, Deserialize)]
 pub struct GetHeadersRes {