fix(providers): use non-descriptive placeholder when stripping images

The image-strip fallback (triggered when a model errors on image input)
replaced image_url blocks with [image: <path>] or [image omitted]. Both
read like a live, available image to the LLM, causing it to:

1. hallucinate about image contents it never received
2. attempt read_file on the leaked server path
3. expose internal file paths to the model

Replace with an explicit '[Image not delivered to model — do not describe
or reference it]' placeholder that tells the LLM the image was stripped.

Fixes #4345

nanobot/providers/base.py

@@ -15,8 +15,6 @@ from typing import Any
 import json_repair
 from loguru import logger
 
-from nanobot.utils.helpers import image_placeholder_text
-
 STREAM_IDLE_TIMEOUT_ENV = "NANOBOT_STREAM_IDLE_TIMEOUT_S"
 DEFAULT_STREAM_IDLE_TIMEOUT_S = 90.0
 MAX_STREAM_IDLE_TIMEOUT_S = 3600.0
@@ -564,8 +562,10 @@ class LLMProvider(ABC):
                 new_content = []
                 for b in content:
                     if isinstance(b, dict) and b.get("type") == "image_url":
-                        path = (b.get("_meta") or {}).get("path", "")
-                        placeholder = image_placeholder_text(path, empty="[image omitted]")
+                        placeholder = (
+                            "[Image not delivered to model — "
+                            "do not describe or reference it]"
+                        )
                         new_content.append({"type": "text", "text": placeholder})
                         found = True
                     else:
@@ -589,8 +589,10 @@ class LLMProvider(ABC):
             if isinstance(content, list):
                 for i, b in enumerate(content):
                     if isinstance(b, dict) and b.get("type") == "image_url":
-                        path = (b.get("_meta") or {}).get("path", "")
-                        placeholder = image_placeholder_text(path, empty="[image omitted]")
+                        placeholder = (
+                            "[Image not delivered to model — "
+                            "do not describe or reference it]"
+                        )
                         content[i] = {"type": "text", "text": placeholder}
                         found = True
         return found