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/mock.rs

@@ -120,6 +120,8 @@ impl MockResponse {
                 prompt_tokens: 100,
                 completion_tokens: content.len() as u32 / 4,
                 total_tokens: 100 + content.len() as u32 / 4,
+                cache_creation_tokens: 0,
+                cache_read_tokens: 0,
             },
         }
     }
@@ -139,6 +141,8 @@ impl MockResponse {
                 prompt_tokens: 100,
                 completion_tokens: total_content.len() as u32 / 4,
                 total_tokens: 100 + total_content.len() as u32 / 4,
+                cache_creation_tokens: 0,
+                cache_read_tokens: 0,
             },
         }
     }
@@ -155,6 +159,8 @@ impl MockResponse {
                 prompt_tokens: 100,
                 completion_tokens: 50,
                 total_tokens: 150,
+                cache_creation_tokens: 0,
+                cache_read_tokens: 0,
             },
         }
     }
@@ -172,6 +178,8 @@ impl MockResponse {
                 prompt_tokens: 100,
                 completion_tokens: 50 + text.len() as u32 / 4,
                 total_tokens: 150 + text.len() as u32 / 4,
+                cache_creation_tokens: 0,
+                cache_read_tokens: 0,
             },
         }
     }
@@ -192,6 +200,8 @@ impl MockResponse {
                 prompt_tokens: 100,
                 completion_tokens: 100,
                 total_tokens: 200,
+                cache_creation_tokens: 0,
+                cache_read_tokens: 0,
             },
         }
     }
@@ -215,6 +225,8 @@ impl MockResponse {
                 prompt_tokens: 100,
                 completion_tokens: full_content.len() as u32 / 4,
                 total_tokens: 100 + full_content.len() as u32 / 4,
+                cache_creation_tokens: 0,
+                cache_read_tokens: 0,
             },
         }
     }
@@ -230,6 +242,8 @@ impl MockResponse {
                 prompt_tokens: 100,
                 completion_tokens: content.len() as u32 / 4,
                 total_tokens: 100 + content.len() as u32 / 4,
+                cache_creation_tokens: 0,
+                cache_read_tokens: 0,
             },
         }
     }