feat(message): the image content block

Providers map images to per-backend content blocks, so the block rides
the message seam beside the tool blocks and round-trips through session
checkpoints. Untagged serde keeps old payloads loading; the new variant
sits last so the field sets disambiguate. Exhaustive matches on Content
across the adapters and views gained a mechanical Image arm; sending
images is a later effort.

crates/rhema-claw/src/handler.rs

@@ -503,7 +503,7 @@ pub(crate) fn final_text(message: &Message) -> Option<String> {
         .iter()
         .filter_map(|content| match content {
             Content::Text(text) => Some(text.as_str()),
-            Content::ToolCall(_) | Content::ToolResult(_) => None,
+            Content::ToolCall(_) | Content::ToolResult(_) | Content::Image(_) => None,
         })
         .collect::<Vec<_>>()
         .join("");

crates/rhema-claw/src/interactive.rs

@@ -474,7 +474,7 @@ fn final_text(message: &Message) -> Option<String> {
         .iter()
         .filter_map(|content| match content {
             Content::Text(text) => Some(text.as_str()),
-            Content::ToolCall(_) | Content::ToolResult(_) => None,
+            Content::ToolCall(_) | Content::ToolResult(_) | Content::Image(_) => None,
         })
         .collect::<Vec<_>>()
         .join("");