fix(agents): replace anyOf with string in image tool schema

Anthropic's API rejects `anyOf` in `input_schema`, causing all Claude
requests to fail when the image tool is registered. Replace
`Type.Union([Type.String(), Type.Array(Type.String())])` with
`Type.String()` — the execute handler already normalizes both string
and array inputs, so this is schema-only.

Fixes #18551

src/agents/tools/image-tool.ts

@@ -376,7 +376,7 @@ export function createImageTool(options?: {
     description,
     parameters: Type.Object({
       prompt: Type.Optional(Type.String()),
-      image: Type.Union([Type.String(), Type.Array(Type.String())]),
+      image: Type.String({ description: "Image path or URL (pass multiple as comma-separated)" }),
       model: Type.Optional(Type.String()),
       maxBytesMb: Type.Optional(Type.Number()),
       maxImages: Type.Optional(Type.Number()),