chore: import upstream snapshot with attribution

examples/config-multi-turn/README.md

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+# config-multi-turn (Multiple Turn Conversation)
+
+You can run this example with:
+
+```bash
+npx promptfoo@latest init --example config-multi-turn
+cd config-multi-turn
+```
+
+## Usage
+
+To get started, set your OPENAI_API_KEY environment variable.
+
+Next, have a look at prompt.json and edit promptfooconfig.yaml. The prompt uses a special built-in variable `_conversation` that has the following signature:
+
+```ts
+type Completion = {
+  prompt: string | object;
+  input: string;
+  output: string;
+};
+
+type Conversation = Completion[];
+```
+
+When looping through `_conversation`, use `completion.prompt` in the Nunjucks prompt template to use the previous outputs. For example, `completion.prompt[completion.prompt.length - 1].content` is the last user message sent in a chat-formatted prompt.
+
+`completion.input` is the last part of the prompt. In a chat-formatted conversation, it will be equal to `completion.prompt[completion.prompt.length - 1].content`. In other conversations, it will be equal to `completion.prompt[completion.prompt.length - 1]`.
+
+Use `completion.output` to get the assistant's response to that message.
+
+Then run:
+
+```bash
+promptfoo eval
+```
+
+Afterwards, you can view the results by running `promptfoo view`

examples/config-multi-turn/prompt.json

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+[
+  {
+    "role": "system",
+    "content": "{{ system_message }}"
+  },
+  {% for completion in _conversation %}
+    {
+      "role": "user",
+      "content": "{{ completion.prompt[completion.prompt.length - 1].content }}"
+    },
+    {
+      "role": "assistant",
+      "content": "{{ completion.output }}"
+    },
+  {% endfor %}
+  {
+    "role": "user",
+    "content": "{{ question }}"
+  }
+]

examples/config-multi-turn/promptfooconfig.yaml

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+# yaml-language-server: $schema=https://promptfoo.dev/config-schema.json
+description: Multi-turn conversation evaluation and testing
+
+prompts:
+  - id: file://prompt.json
+    label: multi-turn-conversation
+providers:
+  - openai:gpt-4.1-mini
+
+# Set up the system message for all tests
+defaultTest:
+  vars:
+    system_message: You are a helpful assistant
+
+# Test out conversations with multiple turns
+tests:
+  # First conversation about Facebook
+  - vars:
+      question: Who founded Facebook?
+    metadata:
+      conversationId: facebook_thread
+  - vars:
+      question: Where does he live?
+    metadata:
+      conversationId: facebook_thread
+  - vars:
+      question: Which state is that in?
+    metadata:
+      conversationId: facebook_thread
+
+  # Second conversation about weather (runs independently)
+  - vars:
+      question: What's the weather like in San Francisco?
+    metadata:
+      conversationId: weather_thread
+  - vars:
+      question: Do I need an umbrella today?
+    metadata:
+      conversationId: weather_thread
+
+  # Third conversation about food (runs independently)
+  - vars:
+      question: Whats a good recipe for pasta?
+    metadata:
+      conversationId: food_thread
+  - vars:
+      question: How long should I cook it?
+    metadata:
+      conversationId: food_thread