* 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.