feat: add view_image tool and optimize web fetch tools

Add image viewing capability for vision-enabled models with ViewImageMiddleware and view_image_tool. Limit web_fetch tool output to 4096 characters to prevent excessive content. Update model config to support vision capability flag.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

backend/src/agents/thread_state.py

@@ -1,4 +1,4 @@
-from typing import NotRequired, TypedDict
+from typing import Annotated, NotRequired, TypedDict
 
 from langchain.agents import AgentState
 
@@ -13,10 +13,43 @@ class ThreadDataState(TypedDict):
     outputs_path: NotRequired[str | None]
 
 
+class ViewedImageData(TypedDict):
+    base64: str
+    mime_type: str
+
+
+def merge_artifacts(existing: list[str] | None, new: list[str] | None) -> list[str]:
+    """Reducer for artifacts list - merges and deduplicates artifacts."""
+    if existing is None:
+        return new or []
+    if new is None:
+        return existing
+    # Use dict.fromkeys to deduplicate while preserving order
+    return list(dict.fromkeys(existing + new))
+
+
+def merge_viewed_images(existing: dict[str, ViewedImageData] | None, new: dict[str, ViewedImageData] | None) -> dict[str, ViewedImageData]:
+    """Reducer for viewed_images dict - merges image dictionaries.
+
+    Special case: If new is an empty dict {}, it clears the existing images.
+    This allows middlewares to clear the viewed_images state after processing.
+    """
+    if existing is None:
+        return new or {}
+    if new is None:
+        return existing
+    # Special case: empty dict means clear all viewed images
+    if len(new) == 0:
+        return {}
+    # Merge dictionaries, new values override existing ones for same keys
+    return {**existing, **new}
+
+
 class ThreadState(AgentState):
     sandbox: NotRequired[SandboxState | None]
     thread_data: NotRequired[ThreadDataState | None]
     title: NotRequired[str | None]
-    artifacts: NotRequired[list[str] | None]
+    artifacts: Annotated[list[str], merge_artifacts]
     todos: NotRequired[list | None]
     uploaded_files: NotRequired[list[dict] | None]
+    viewed_images: Annotated[dict[str, ViewedImageData], merge_viewed_images]  # image_path -> {base64, mime_type}