feat: Allow AgentContext update consumer in BaseAgent and add beforeStream to TextAgentResponseHandler

This commit introduces the following changes:

- **BaseAgent:** Added a mechanism to allow consumers to update the AgentContext after post-processing a split. This provides more flexibility for downstream components to react to the processed data.
- **TextAgentResponseHandler:** Introduced a `beforeStream` consumer that allows updating the AgentContext before the stream begins.  This enables pre-processing or initialization steps within the response handler.  The constructor has been updated to accept this new consumer.  Also fixed a bug where the chunkKey and updateKey were not being removed after processing.

These changes enhance the extensibility and configurability of the agent processing pipeline.

src/main/java/de/extio/lmlib/agent/BaseAgent.java

@@ -142,6 +142,9 @@ public interface BaseAgent {
 					}
 					
 					this.postProcess(split.context());
+					if (split.context().getAgentContextUpdateConsumer() != null) {
+						split.context().getAgentContextUpdateConsumer().accept(split.context());
+					}
 					
 					finishedSplits.add(split);
 				}