copilot: Fix vision request detection for follow-up messages (#31760)

Previously, the vision request header was only set if the last message
in a thread contained an image. This caused 400 errors from the Copilot
API when sending follow-up messages in a thread that contained images in
earlier messages.

Modified the `is_vision_request` check to scan all messages in a thread
for image content instead of just the last one, ensuring the proper
header is set for the entire conversation.

Added a unit test to verify all cases function correctly.

Release Notes:

- Fix GitHub Copilot chat provider error when sending follow-up messages
in threads containing images

crates/copilot/src/copilot_chat.rs

@@ -581,7 +581,7 @@ async fn stream_completion(
     api_key: String,
     request: Request,
 ) -> Result<BoxStream<'static, Result<ResponseEvent>>> {
-    let is_vision_request = request.messages.last().map_or(false, |message| match message {
+    let is_vision_request = request.messages.iter().any(|message| match message {
         ChatMessage::User { content }
         | ChatMessage::Assistant { content, .. }
         | ChatMessage::Tool { content, .. } => {
@@ -736,4 +736,116 @@ mod tests {
         assert_eq!(schema.data[0].id, "gpt-4");
         assert_eq!(schema.data[1].id, "claude-3.7-sonnet");
     }
+
+    #[test]
+    fn test_vision_request_detection() {
+        fn message_contains_image(message: &ChatMessage) -> bool {
+            match message {
+                ChatMessage::User { content }
+                | ChatMessage::Assistant { content, .. }
+                | ChatMessage::Tool { content, .. } => {
+                    matches!(content, ChatMessageContent::Multipart(parts) if
+                        parts.iter().any(|part| matches!(part, ChatMessagePart::Image { .. })))
+                }
+                _ => false,
+            }
+        }
+
+        // Helper function to detect if a request is a vision request
+        fn is_vision_request(request: &Request) -> bool {
+            request.messages.iter().any(message_contains_image)
+        }
+
+        let request_with_image_in_last = Request {
+            intent: true,
+            n: 1,
+            stream: true,
+            temperature: 0.1,
+            model: "claude-3.7-sonnet".to_string(),
+            messages: vec![
+                ChatMessage::User {
+                    content: ChatMessageContent::Plain("Hello".to_string()),
+                },
+                ChatMessage::Assistant {
+                    content: ChatMessageContent::Plain("How can I help?".to_string()),
+                    tool_calls: vec![],
+                },
+                ChatMessage::User {
+                    content: ChatMessageContent::Multipart(vec![
+                        ChatMessagePart::Text {
+                            text: "What's in this image?".to_string(),
+                        },
+                        ChatMessagePart::Image {
+                            image_url: ImageUrl {
+                                url: "data:image/png;base64,abc123".to_string(),
+                            },
+                        },
+                    ]),
+                },
+            ],
+            tools: vec![],
+            tool_choice: None,
+        };
+
+        let request_with_image_in_earlier = Request {
+            intent: true,
+            n: 1,
+            stream: true,
+            temperature: 0.1,
+            model: "claude-3.7-sonnet".to_string(),
+            messages: vec![
+                ChatMessage::User {
+                    content: ChatMessageContent::Plain("Hello".to_string()),
+                },
+                ChatMessage::User {
+                    content: ChatMessageContent::Multipart(vec![
+                        ChatMessagePart::Text {
+                            text: "What's in this image?".to_string(),
+                        },
+                        ChatMessagePart::Image {
+                            image_url: ImageUrl {
+                                url: "data:image/png;base64,abc123".to_string(),
+                            },
+                        },
+                    ]),
+                },
+                ChatMessage::Assistant {
+                    content: ChatMessageContent::Plain("I see a cat in the image.".to_string()),
+                    tool_calls: vec![],
+                },
+                ChatMessage::User {
+                    content: ChatMessageContent::Plain("What color is it?".to_string()),
+                },
+            ],
+            tools: vec![],
+            tool_choice: None,
+        };
+
+        let request_with_no_images = Request {
+            intent: true,
+            n: 1,
+            stream: true,
+            temperature: 0.1,
+            model: "claude-3.7-sonnet".to_string(),
+            messages: vec![
+                ChatMessage::User {
+                    content: ChatMessageContent::Plain("Hello".to_string()),
+                },
+                ChatMessage::Assistant {
+                    content: ChatMessageContent::Plain("How can I help?".to_string()),
+                    tool_calls: vec![],
+                },
+                ChatMessage::User {
+                    content: ChatMessageContent::Plain("Tell me about Rust.".to_string()),
+                },
+            ],
+            tools: vec![],
+            tool_choice: None,
+        };
+
+        assert!(is_vision_request(&request_with_image_in_last));
+        assert!(is_vision_request(&request_with_image_in_earlier));
+
+        assert!(!is_vision_request(&request_with_no_images));
+    }
 }