Fix availability handling with MQTT retain flag

Added retain flag support to MQTT messages to properly handle device availability in Home Assistant. Both the availability "online" publish and the last will "offline" message now use retain=true, ensuring HA always sees the current device status even when subscribing after the messages were sent.

Changes:
- Added will_retain field to embedded_mqtt::ConnectParams
- Set retain=true for availability publish using publish_with()
- Set will_retain=true in connect params for last will message

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

2 files changed, 11 insertions, 2 deletions

diff --git a/embedded-mqtt/src/lib.rs b/embedded-mqtt/src/lib.rs
index 096af63..7367b53 100644
--- a/embedded-mqtt/src/lib.rs
+++ b/embedded-mqtt/src/lib.rs
@@ -71,6 +71,7 @@ where
 pub struct ConnectParams<'a> {
     pub will_topic: Option<&'a str>,
     pub will_payload: Option<&'a [u8]>,
+    pub will_retain: bool,
     pub username: Option<&'a str>,
     pub password: Option<&'a [u8]>,
     pub keepalive: Option<u16>,
@@ -197,7 +198,7 @@ where
                 password: params.password,
                 will_topic: params.will_topic,
                 will_payload: params.will_payload,
-                will_retain: false,
+                will_retain: params.will_retain,
                 keepalive: None,
             },
         );
diff --git a/src/lib.rs b/src/lib.rs
index 3353663..ca2ab82 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -541,6 +541,7 @@ impl<'a> Device<'a> {
         let connect_params = embedded_mqtt::ConnectParams {
             will_topic: Some(availability_topic),
             will_payload: Some(NOT_AVAILABLE_PAYLOAD.as_bytes()),
+            will_retain: true,
             ..Default::default()
         };
         if let Err(err) = client
@@ -665,7 +666,14 @@ impl<'a> Device<'a> {
         }
 
         if let Err(err) = client
-            .publish(availability_topic, AVAILABLE_PAYLOAD.as_bytes())
+            .publish_with(
+                availability_topic,
+                AVAILABLE_PAYLOAD.as_bytes(),
+                embedded_mqtt::PublishParams {
+                    retain: true,
+                    ..Default::default()
+                },
+            )
             .await
         {
             crate::log::error!(