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/lib.rs

@@ -196,6 +196,12 @@ pub struct Usage {
     pub prompt_tokens: u32,
     pub completion_tokens: u32,
     pub total_tokens: u32,
+    /// Tokens written to cache (Anthropic: cache_creation_input_tokens)
+    #[serde(default)]
+    pub cache_creation_tokens: u32,
+    /// Tokens read from cache (Anthropic: cache_read_input_tokens, OpenAI: cached_tokens)
+    #[serde(default)]
+    pub cache_read_tokens: u32,
 }
 
 pub type CompletionStream = tokio_stream::wrappers::ReceiverStream<Result<CompletionChunk>>;