Add prompt cache statistics tracking to /stats command

- Extend Usage struct with cache_creation_tokens and cache_read_tokens fields
- Parse Anthropic cache_creation_input_tokens and cache_read_input_tokens
- Parse OpenAI prompt_tokens_details.cached_tokens for automatic prefix caching
- Add CacheStats struct to Agent for cumulative tracking across API calls
- Add "Prompt Cache Statistics" section to /stats output showing:
  - API call count and cache hit count
  - Hit rate percentage
  - Total input tokens and cache read/creation tokens
  - Cache efficiency (% of input served from cache)
- Update all provider implementations and test files

crates/g3-providers/src/databricks.rs

@@ -763,6 +763,8 @@ impl LLMProvider for DatabricksProvider {
             prompt_tokens: databricks_response.usage.prompt_tokens,
             completion_tokens: databricks_response.usage.completion_tokens,
             total_tokens: databricks_response.usage.total_tokens,
+            cache_creation_tokens: 0, // Databricks doesn't support prompt caching
+            cache_read_tokens: 0,
         };
 
         debug!(