fix: system prompt formatting for Vertex AI

Prior to this commit, system prompt messages were included
by the `KurtVertexAI` as part of the `content` stream of messages.
However, VertexAI requires system messages to be specified separately.
This commit makes that change.

packages/kurt-vertex-ai/spec/generateNaturalLanguage.spec.ts

@@ -12,6 +12,16 @@ describe("KurtVertexAI generateNaturalLanguage", () => {
     expect(result.text).toEqual("Hello! 👋  😊\n")
   })
 
+  test("properly formats a system prompt for Vertex AI", async () => {
+    const result = await snapshotAndMock((kurt) =>
+      kurt.generateNaturalLanguage({
+        systemPrompt: "Don't be evil.", // sometimes Google needs to remind themselves
+        prompt: "Say hello!",
+      })
+    )
+    expect(result.text).toEqual("Hello! 👋  😊\n")
+  })
+
   test("writes a haiku with high temperature", async () => {
     const result = await snapshotAndMock((kurt) =>
       kurt.generateNaturalLanguage({

packages/kurt-vertex-ai/spec/snapshots/KurtVertexAI_generateNaturalLanguage_properly_formats_a_system_prompt_for_Vertex_AI.yaml

@@ -0,0 +1,43 @@
+step1Request:
+  generationConfig:
+    maxOutputTokens: 4096
+    temperature: 0.5
+    topP: 0.95
+  contents:
+    - role: user
+      parts:
+        - text: Say hello!
+  systemInstruction:
+    role: system
+    parts:
+      - text: Don't be evil.
+step2RawChunks:
+  - candidates:
+      - content:
+          role: model
+          parts:
+            - text: Hello
+        index: 0
+    usageMetadata: {}
+  - candidates:
+      - content:
+          role: model
+          parts:
+            - text: |
+                ! 👋  😊
+        finishReason: STOP
+        index: 0
+    usageMetadata:
+      promptTokenCount: 9
+      candidatesTokenCount: 6
+      totalTokenCount: 15
+step3KurtEvents:
+  - chunk: Hello
+  - chunk: |
+      ! 👋  😊
+  - finished: true
+    text: |
+      Hello! 👋  😊
+    metadata:
+      totalInputTokens: 9
+      totalOutputTokens: 6

packages/kurt-vertex-ai/src/KurtVertexAI.ts

@@ -91,13 +91,26 @@ export class KurtVertexAI
       model: this.options.model,
     }) as VertexAIGenerativeModel
 
+    // VertexAI requires that system messages be sent as a single message,
+    // so we filter them out from the main messages array to send separately.
+    const normalMessages = options.messages.filter((m) => m.role !== "system")
+    const systemMessages = options.messages.filter((m) => m.role === "system")
+    const singleSystemMessage: VertexAIMessage | undefined =
+      systemMessages.length === 0
+        ? undefined
+        : {
+            role: "system",
+            parts: systemMessages.flatMap((m) => m.parts),
+          }
+
     const req: VertexAIRequest = {
       generationConfig: {
         maxOutputTokens: options.sampling.maxOutputTokens,
         temperature: options.sampling.temperature,
         topP: options.sampling.topP,
       },
-      contents: options.messages,
+      contents: normalMessages,
+      systemInstruction: singleSystemMessage,
     }
 
     const tools = Object.values(options.tools)