feat(translators): add token counting support for Claude and Gemini responses

- Implemented `TokenCount` transform method across translators to calculate token usage.
- Integrated token counting logic into executor pipelines for Claude, Gemini, and CLI translators.
- Added corresponding API endpoints and handlers (`/messages/count_tokens`) for token usage retrieval.
- Enhanced translation registry to support `TokenCount` functionality alongside existing response types.

internal/translator/claude/gemini/init.go

@@ -12,8 +12,9 @@ func init() {
 		Claude,
 		ConvertGeminiRequestToClaude,
 		interfaces.TranslateResponse{
-			Stream:    ConvertClaudeResponseToGemini,
-			NonStream: ConvertClaudeResponseToGeminiNonStream,
+			Stream:     ConvertClaudeResponseToGemini,
+			NonStream:  ConvertClaudeResponseToGeminiNonStream,
+			TokenCount: GeminiTokenCount,
 		},
 	)
 }