**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/runtime/executor/antigravity_executor.go

@@ -70,7 +70,7 @@ func (e *AntigravityExecutor) Execute(ctx context.Context, auth *cliproxyauth.Au
 	defer reporter.trackFailure(ctx, &err)
 
 	from := opts.SourceFormat
-	to := sdktranslator.FromString("gemini-cli")
+	to := sdktranslator.FromString("antigravity")
 	translated := sdktranslator.TranslateRequest(from, to, req.Model, bytes.Clone(req.Payload), false)
 
 	httpReq, errReq := e.buildRequest(ctx, auth, token, req.Model, translated, false, opts.Alt)
@@ -127,7 +127,7 @@ func (e *AntigravityExecutor) ExecuteStream(ctx context.Context, auth *cliproxya
 	defer reporter.trackFailure(ctx, &err)
 
 	from := opts.SourceFormat
-	to := sdktranslator.FromString("gemini-cli")
+	to := sdktranslator.FromString("antigravity")
 	translated := sdktranslator.TranslateRequest(from, to, req.Model, bytes.Clone(req.Payload), true)
 
 	httpReq, errReq := e.buildRequest(ctx, auth, token, req.Model, translated, true, opts.Alt)