gemini: Pass system prompt as system instructions (#28793)

https://ai.google.dev/gemini-api/docs/text-generation#system-instructions

Release Notes:

- agent: Improve performance of Gemini models

crates/google_ai/src/google_ai.rs

@@ -125,6 +125,7 @@ pub struct GenerateContentRequest {
     #[serde(default, skip_serializing_if = "String::is_empty")]
     pub model: String,
     pub contents: Vec<Content>,
+    pub system_instructions: Option<SystemInstructions>,
     pub generation_config: Option<GenerationConfig>,
     pub safety_settings: Option<Vec<SafetySetting>>,
     #[serde(skip_serializing_if = "Option::is_none")]
@@ -159,6 +160,12 @@ pub struct Content {
     pub role: Role,
 }
 
+#[derive(Debug, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct SystemInstructions {
+    pub parts: Vec<Part>,
+}
+
 #[derive(Debug, PartialEq, Deserialize, Serialize)]
 #[serde(rename_all = "camelCase")]
 pub enum Role {

crates/language_models/src/provider/google.rs

@@ -3,14 +3,16 @@ use collections::BTreeMap;
 use credentials_provider::CredentialsProvider;
 use editor::{Editor, EditorElement, EditorStyle};
 use futures::{FutureExt, Stream, StreamExt, future::BoxFuture};
-use google_ai::{FunctionDeclaration, GenerateContentResponse, Part, UsageMetadata};
+use google_ai::{
+    FunctionDeclaration, GenerateContentResponse, Part, SystemInstructions, UsageMetadata,
+};
 use gpui::{
     AnyView, App, AsyncApp, Context, Entity, FontStyle, Subscription, Task, TextStyle, WhiteSpace,
 };
 use http_client::HttpClient;
 use language_model::{
     AuthenticateError, LanguageModelCompletionEvent, LanguageModelToolSchemaFormat,
-    LanguageModelToolUse, LanguageModelToolUseId, StopReason,
+    LanguageModelToolUse, LanguageModelToolUseId, MessageContent, StopReason,
 };
 use language_model::{
     LanguageModel, LanguageModelId, LanguageModelName, LanguageModelProvider,
@@ -359,17 +361,11 @@ impl LanguageModel for GoogleLanguageModel {
 }
 
 pub fn into_google(
-    request: LanguageModelRequest,
+    mut request: LanguageModelRequest,
     model: String,
 ) -> google_ai::GenerateContentRequest {
-    google_ai::GenerateContentRequest {
-        model,
-        contents: request
-            .messages
-            .into_iter()
-            .map(|message| google_ai::Content {
-                parts: message
-                    .content
+    fn map_content(content: Vec<MessageContent>) -> Vec<Part> {
+        content
             .into_iter()
             .filter_map(|content| match content {
                 language_model::MessageContent::Text(text) => {
@@ -400,7 +396,30 @@ pub fn into_google(
                     }),
                 ),
             })
-                    .collect(),
+            .collect()
+    }
+
+    let system_instructions = if request
+        .messages
+        .first()
+        .map_or(false, |msg| matches!(msg.role, Role::System))
+    {
+        let message = request.messages.remove(0);
+        Some(SystemInstructions {
+            parts: map_content(message.content),
+        })
+    } else {
+        None
+    };
+
+    google_ai::GenerateContentRequest {
+        model,
+        system_instructions,
+        contents: request
+            .messages
+            .into_iter()
+            .map(|message| google_ai::Content {
+                parts: map_content(message.content),
                 role: match message.role {
                     Role::User => google_ai::Role::User,
                     Role::Assistant => google_ai::Role::Model,