**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/usage_helpers.go

@@ -453,6 +453,10 @@ func StripUsageMetadataFromJSON(rawJSON []byte) ([]byte, bool) {
 		usageMetadata = gjson.GetBytes(jsonBytes, "response.usageMetadata")
 	}
 
+	if hasNonZeroUsageMetadata(usageMetadata) {
+		return rawJSON, false
+	}
+
 	if !usageMetadata.Exists() {
 		return rawJSON, false
 	}
@@ -475,3 +479,14 @@ func StripUsageMetadataFromJSON(rawJSON []byte) ([]byte, bool) {
 
 	return cleaned, changed
 }
+
+// hasNonZeroUsageMetadata checks if any usage token counts are present.
+func hasNonZeroUsageMetadata(node gjson.Result) bool {
+	if !node.Exists() {
+		return false
+	}
+	return node.Get("totalTokenCount").Int() > 0 ||
+		node.Get("promptTokenCount").Int() > 0 ||
+		node.Get("candidatesTokenCount").Int() > 0 ||
+		node.Get("thoughtsTokenCount").Int() > 0
+}