feat(transmission): add length_in_bytes method to Protocol300Transmission

Add a new method to calculate the byte length of Protocol300Transmission
variants without converting to bytes. The method returns:
- 1 byte for control characters (Ack, Enq, Eot, Nack)
- 3 bytes for Init
- telegram_length + 3 bytes for Message

Include comprehensive unit tests for all transmission variants to ensure
correct length calculation.

src/transmission.rs

@@ -61,6 +61,16 @@ impl Protocol300Transmission {
             Self::Message(protocol300_message) => protocol300_message.to_bytes(),
         }
     }
+
+    /// Returns the length in bytes of the transmission
+    #[must_use]
+    pub fn length_in_bytes(&self) -> usize {
+        match self {
+            Self::Ack | Self::Enq | Self::Eot | Self::Nack => 1,
+            Self::Init => 3,
+            Self::Message(protocol300_message) => (protocol300_message.telegram_length + 3).into(),
+        }
+    }
 }
 
 /// Errors that can occur when getting a Protocol 300 Transmission from bytes
@@ -155,4 +165,44 @@ mod tests {
         };
         assert_eq!(Protocol300Transmission::Message(message).to_bytes(), &[0x41, 0x09, 0x01, 0x01, 0x55, 0x25, 0x04, 0x07, 0x01, 0x27, 0x11, 0xC9]);
     }
+
+    #[test]
+    fn length_in_bytes_ack() {
+        assert_eq!(Protocol300Transmission::Ack.length_in_bytes(), 1);
+    }
+
+    #[test]
+    fn length_in_bytes_enq() {
+        assert_eq!(Protocol300Transmission::Enq.length_in_bytes(), 1);
+    }
+
+    #[test]
+    fn length_in_bytes_eot() {
+        assert_eq!(Protocol300Transmission::Eot.length_in_bytes(), 1);
+    }
+
+    #[test]
+    fn length_in_bytes_nack() {
+        assert_eq!(Protocol300Transmission::Nack.length_in_bytes(), 1);
+    }
+
+    #[test]
+    fn length_in_bytes_init() {
+        assert_eq!(Protocol300Transmission::Init.length_in_bytes(), 3);
+    }
+
+    #[test]
+    fn length_in_bytes_message() {
+        let message = Protocol300Message{
+            data_address: 0x5525,
+            telegram_length: 0x09,
+            function_code: FunctionCode::VirtualREAD,
+            message_identifier: MessageIdentifier::Response,
+            data_length: 0x04,
+            data: vec![0x07, 0x01, 0x27, 0x11],
+            checksum: 0xC9
+        };
+        // telegram_length is 0x09, so expected length is 0x09 + 3 = 12
+        assert_eq!(Protocol300Transmission::Message(message).length_in_bytes(), 12);
+    }
 }