fix(bom): add langchain4j-reactor dependency (langchain4j#1838)

## Change:

Added `langchain4j-reactor` to the `langchain4j-bom` module to avoid
needing to specify the version manually when importing
`langchain4j-reactor`, allowing it to be managed by the BOM.


<img width="1090" alt="image"
src="https://github.com/user-attachments/assets/06d5a67f-bcea-49b2-a116-939a842d9454">

langchain4j-bom/pom.xml

@@ -31,6 +31,12 @@
                 <version>${project.version}</version>
             </dependency>
 
+            <dependency>
+                <groupId>dev.langchain4j</groupId>
+                <artifactId>langchain4j-reactor</artifactId>
+                <version>${project.version}</version>
+            </dependency>
+
             <dependency>
                 <groupId>dev.langchain4j</groupId>
                 <artifactId>langchain4j-easy-rag</artifactId>