Add changelog for #3918

changelog/3918.added.md

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+- Wired up `system_instruction` in `BaseOpenAILLMService`, `AnthropicLLMService`, and `AWSBedrockLLMService` so it works as a default system prompt, matching the behavior of the Google services. This enables sharing a single `LLMContext` across multiple LLM services, where each service provides its own system instruction independently.
+
+  ```python
+  llm = OpenAILLMService(
+      api_key=os.getenv("OPENAI_API_KEY"),
+      system_instruction="You are a helpful assistant.",
+  )
+
+  context = LLMContext()
+
+  @transport.event_handler("on_client_connected")
+  async def on_client_connected(transport, client):
+      context.add_message({"role": "user", "content": "Please introduce yourself."})
+      await task.queue_frames([LLMRunFrame()])
+  ```

changelog/3918.other.md

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+- Updated foundational examples to use `system_instruction` on LLM services instead of adding system messages to `LLMContext`.