fix(test): update openrouter image generation assertion for gemini-2.5-flash-image (#23070)

* fix(anthropic/skills): remove ?beta=true query param from Skills API URLs

Beta access is controlled via the anthropic-beta header (already set
to skills-2025-10-02), not a URL query param. The spurious ?beta=true
was causing 500 errors from Anthropic's server.

* fix(test): update openrouter image generation assertion to accept any image format

gemini-2.5-flash-image returns JPEG, not PNG. The assertion was hardcoded
to png after the model was swapped from gemini-2.5-flash-image-preview
(which returned PNG) in commit 34e8e972.

tests/llm_translation/test_openrouter.py

@@ -30,7 +30,7 @@ def test_completion_openrouter_image_generation():
     assert (
         resp.choices[0]
         .message.images[0]["image_url"]["url"]
-        .startswith("data:image/png;base64,")
+        .startswith("data:image/")
     )