fix(antigravity): always include text field in thought parts to prevent Google 500

When Claude sends redacted thinking with empty text, the translator
was omitting the "text" field from thought parts. Google Antigravity
API requires this field, causing 500 "Unknown Error" responses.

Verified: 129/129 error logs with empty thought → 500, 0/97 success
logs had empty thought. After fix: 0 new "Unknown Error" 500s.

internal/translator/antigravity/claude/antigravity_claude_request.go

@@ -148,11 +148,11 @@ func ConvertClaudeRequestToAntigravity(modelName string, inputRawJSON []byte, _
 					}
 
 					// Valid signature, send as thought block
+					// Always include "text" field — Google Antigravity API requires it
+					// even for redacted thinking where the text is empty.
 					partJSON := []byte(`{}`)
 					partJSON, _ = sjson.SetBytes(partJSON, "thought", true)
-						if thinkingText != "" {
 					partJSON, _ = sjson.SetBytes(partJSON, "text", thinkingText)
-						}
 					if signature != "" {
 						partJSON, _ = sjson.SetBytes(partJSON, "thoughtSignature", signature)
 					}