fix(tools): add the native function schema for generate_image

generate_image existed in the prompt sections, the executor and the image_gen
MCP server, but not in FUNCTION_TOOL_SCHEMAS. API models are only sent schemas
from that list, so the tool selector could pick it and the schema filter would
drop it again — the model then improvised text-shaped calls it had no way to
make work.

The existing JSON fallback in function_call_to_tool_block already produces
content the executor parses, so no converter branch is needed. A required-args
entry is, though: without it a prompt-less call falls through to the line
parser and the whole JSON blob becomes the image prompt.

Fixes #5520

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

src/tool_schemas.py

@@ -26,6 +26,9 @@ _REQUIRED_NATIVE_TOOL_ARGS = {
     "write_file": ("path",),
     "edit_file": ("path",),
     "apply_patch": ("patch_text", "patchText", "patch"),
+    # Without this, a prompt-less call falls through to the line parser in
+    # tool_execution and the whole JSON blob becomes the image prompt.
+    "generate_image": ("prompt",),
 }
 
 # ---------------------------------------------------------------------------
@@ -1030,6 +1033,23 @@ FUNCTION_TOOL_SCHEMAS = [
             }
         }
     },
+    {
+        "type": "function",
+        "function": {
+            "name": "generate_image",
+            "description": "Generate an image from a text prompt. Art, illustrations, photos.",
+            "parameters": {
+                "type": "object",
+                "properties": {
+                    "prompt": {"type": "string", "description": "Image description prompt"},
+                    "model": {"type": "string", "description": "Model name (auto-detects if omitted)"},
+                    "size": {"type": "string", "description": "Image size (default 1024x1024)"},
+                    "quality": {"type": "string", "description": "Quality: low, medium, high, auto (default medium)"},
+                },
+                "required": ["prompt"]
+            }
+        }
+    },
     {
         "type": "function",
         "function": {