**feat(translator): add Antigravity translation logic**

- Introduced request and response translation functions to enable compatibility between OpenAI Chat Completions API and Antigravity.
- Registered translation utilities for both streaming and non-streaming scenarios.
- Added support for reasoning content, tool calls, and metadata handling.
- Established request normalization and embedding for Antigravity-compatible payloads.
- Added new fields to `Params` struct for better tracking of finish reasons, usage metadata, and tool usage.
- Refactored handling of response transitions, final events, and state-driven logic in `ConvertAntigravityResponseToClaude`.
- Introduced `appendFinalEvents` and `resolveStopReason` helper functions for cleaner separation of concerns.
- Added `TotalTokenCount` field to `Params` struct for enhanced token tracking.
- Updated token count calculations to fallback on `TotalTokenCount` when specific counts are missing.
- Introduced `hasNonZeroUsageMetadata` function to validate presence of token data in `usage_metadata`.

internal/translator/antigravity/openai/responses/init.go

@@ -0,0 +1,19 @@
+package responses
+
+import (
+	. "github.com/router-for-me/CLIProxyAPI/v6/internal/constant"
+	"github.com/router-for-me/CLIProxyAPI/v6/internal/interfaces"
+	"github.com/router-for-me/CLIProxyAPI/v6/internal/translator/translator"
+)
+
+func init() {
+	translator.Register(
+		OpenaiResponse,
+		Antigravity,
+		ConvertOpenAIResponsesRequestToAntigravity,
+		interfaces.TranslateResponse{
+			Stream:    ConvertAntigravityResponseToOpenAIResponses,
+			NonStream: ConvertAntigravityResponseToOpenAIResponsesNonStream,
+		},
+	)
+}