feat: implement reasoning_tokens tracking and enhanced usage logging

src/providers/gemini.rs

@@ -722,6 +722,10 @@ impl super::Provider for GeminiProvider {
         let reasoning_content = candidate
             .and_then(|c| c.content.parts.iter().find_map(|p| p.thought.clone()));
 
+        let reasoning_tokens = reasoning_content.as_ref()
+            .map(|r| crate::utils::tokens::estimate_completion_tokens(r, &model))
+            .unwrap_or(0);
+
         // Extract function calls → OpenAI tool_calls
         let tool_calls = candidate.and_then(|c| Self::extract_tool_calls(&c.content.parts));
 
@@ -752,6 +756,7 @@ impl super::Provider for GeminiProvider {
             tool_calls,
             prompt_tokens,
             completion_tokens,
+            reasoning_tokens,
             total_tokens,
             cache_read_tokens,
             cache_write_tokens: 0, // Gemini doesn't report cache writes separately
@@ -902,6 +907,7 @@ impl super::Provider for GeminiProvider {
                             super::StreamUsage {
                                 prompt_tokens: u.prompt_token_count,
                                 completion_tokens: u.candidates_token_count,
+                                reasoning_tokens: 0,
                                 total_tokens: u.total_token_count,
                                 cache_read_tokens: u.cached_content_token_count,
                                 cache_write_tokens: 0,