fix(agent): harden multimodal tool result flow

Keep multimodal tool outputs on the native content-block path while
restoring redirect SSRF checks for web_fetch image responses. Also share
image block construction, simplify persisted history sanitization, and
add regression tests for image reads and blocked private redirects.

Made-with: Cursor

nanobot/utils/helpers.py

@@ -1,5 +1,6 @@
 """Utility functions for nanobot."""
 
+import base64
 import json
 import re
 import time
@@ -23,6 +24,19 @@ def detect_image_mime(data: bytes) -> str | None:
     return None
 
 
+def build_image_content_blocks(raw: bytes, mime: str, path: str, label: str) -> list[dict[str, Any]]:
+    """Build native image blocks plus a short text label."""
+    b64 = base64.b64encode(raw).decode()
+    return [
+        {
+            "type": "image_url",
+            "image_url": {"url": f"data:{mime};base64,{b64}"},
+            "_meta": {"path": path},
+        },
+        {"type": "text", "text": label},
+    ]
+
+
 def ensure_dir(path: Path) -> Path:
     """Ensure directory exists, return it."""
     path.mkdir(parents=True, exist_ok=True)