fix: correct LiteLLM environment variable syntax

Changed API key reference from ${ANTHROPIC_API_KEY} to
os.environ/ANTHROPIC_API_KEY to match LiteLLM's documented syntax.

The os.environ/ prefix tells LiteLLM to use os.getenv() to retrieve
the environment variable at runtime, which is the correct way to
reference environment variables in LiteLLM config files.

Reference: https://docs.litellm.ai/docs/proxy/deploy

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

ai/litellm-config.yaml

@@ -2,27 +2,27 @@ model_list:
   - model_name: claude-sonnet-4
     litellm_params:
       model: anthropic/claude-sonnet-4-20250514
-      api_key: ${ANTHROPIC_API_KEY}
+      api_key: os.environ/ANTHROPIC_API_KEY
 
   - model_name: claude-sonnet-4.5
     litellm_params:
       model: anthropic/claude-sonnet-4-5-20250929
-      api_key: ${ANTHROPIC_API_KEY}
+      api_key: os.environ/ANTHROPIC_API_KEY
 
   - model_name: claude-3-5-sonnet
     litellm_params:
       model: anthropic/claude-3-5-sonnet-20241022
-      api_key: ${ANTHROPIC_API_KEY}
+      api_key: os.environ/ANTHROPIC_API_KEY
 
   - model_name: claude-3-opus
     litellm_params:
       model: anthropic/claude-3-opus-20240229
-      api_key: ${ANTHROPIC_API_KEY}
+      api_key: os.environ/ANTHROPIC_API_KEY
 
   - model_name: claude-3-haiku
     litellm_params:
       model: anthropic/claude-3-haiku-20240307
-      api_key: ${ANTHROPIC_API_KEY}
+      api_key: os.environ/ANTHROPIC_API_KEY
 
 litellm_settings:
   drop_params: true