Enable local image prompts without breaking text-only CLI flows

The Rust CLI now recognizes explicit local image references in prompt text,
encodes supported image files as base64, and serializes mixed text/image
content blocks for the API. The request conversion path was kept narrow so
existing runtime/session structures remain stable while prompt mode and user
text conversion gain multimodal support.

Constraint: Must support PNG, JPG/JPEG, GIF, and WebP without adding broad runtime abstractions
Constraint: Existing text-only prompt behavior and API tool flows must keep working unchanged
Rejected: Add only explicit --image CLI flags | does not satisfy auto-detect image refs in prompt text
Rejected: Persist native image blocks in runtime session model | broader refactor than needed for prompt support
Confidence: high
Scope-risk: moderate
Reversibility: clean
Directive: Keep image parsing scoped to outbound user prompt adaptation unless session persistence truly needs multimodal history
Tested: cargo fmt --all; cargo clippy --workspace --all-targets -- -D warnings; cargo test --workspace
Not-tested: Live remote multimodal request against Anthropic API

rust/crates/api/tests/client_integration.rs

@@ -4,8 +4,8 @@ use std::time::Duration;
 
 use api::{
     AnthropicClient, ApiError, ContentBlockDelta, ContentBlockDeltaEvent, ContentBlockStartEvent,
-    InputContentBlock, InputMessage, MessageDeltaEvent, MessageRequest, OutputContentBlock,
-    StreamEvent, ToolChoice, ToolDefinition,
+    ImageSource, InputContentBlock, InputMessage, MessageDeltaEvent, MessageRequest,
+    OutputContentBlock, StreamEvent, ToolChoice, ToolDefinition,
 };
 use serde_json::json;
 use tokio::io::{AsyncReadExt, AsyncWriteExt};
@@ -75,6 +75,39 @@ async fn send_message_posts_json_and_parses_response() {
     assert_eq!(body["tool_choice"]["type"], json!("auto"));
 }
 
+#[test]
+fn image_content_blocks_serialize_with_base64_source() {
+    let request = MessageRequest {
+        model: "claude-3-7-sonnet-latest".to_string(),
+        max_tokens: 64,
+        messages: vec![InputMessage {
+            role: "user".to_string(),
+            content: vec![InputContentBlock::Image {
+                source: ImageSource {
+                    kind: "base64".to_string(),
+                    media_type: "image/png".to_string(),
+                    data: "AQID".to_string(),
+                },
+            }],
+        }],
+        system: None,
+        tools: None,
+        tool_choice: None,
+        stream: false,
+    };
+
+    let json = serde_json::to_value(request).expect("request should serialize");
+    assert_eq!(json["messages"][0]["content"][0]["type"], json!("image"));
+    assert_eq!(
+        json["messages"][0]["content"][0]["source"],
+        json!({
+            "type": "base64",
+            "media_type": "image/png",
+            "data": "AQID"
+        })
+    );
+}
+
 #[tokio::test]
 async fn stream_message_parses_sse_events_with_tool_use() {
     let state = Arc::new(Mutex::new(Vec::<CapturedRequest>::new()));