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What happened? When calling `GET /v1/skills` on the LiteLLM proxy (v1.90.0) without a valid ANTHROPIC_API_KEY, the endpoint returns HTTP 500 with a Pydantic ValidationError instead of gracefully returning an empty skills list.

The error occurs in two scenarios: 1. No ANTHROPIC_API_KEY set: `ValueError: ANTHROPIC_API_KEY or ANTHROPIC_AUTH_TOKEN is required for Skills API` 2. Invalid ANTHROPIC_API_KEY set: `pydantic_core._pydantic_core.ValidationError: 1 validation error for ListSkillsResponse data Field required`

This breaks clients that automatically call `/v1/skills` on startup (e.g., pi-agent with pi-provider-litellm).

Steps to Reproduce 1. Start LiteLLM proxy WITHOUT `ANTHROPIC_API_KEY` set (or with a fake key like `sk-dummy`) 2. Call `GET /v1/skills` with valid LiteLLM auth 3. Receive HTTP 500

Relevant log output ``` pydantic_core._pydantic_core.ValidationError: 1 validation error for ListSkillsResponse data Field required [type=missing, input_value={'type': 'error', 'error'...}] File "/app/.venv/lib/python3.13/site-packages/litellm/skills/main.py", line 316,…