fix(antigravity): prevent corrupted thought signature when switching models

When switching from Claude models (e.g., Opus 4.5) to Gemini models
(e.g., Flash) mid-conversation via Antigravity OAuth, the client-provided
thinking signatures from Claude would cause "Corrupted thought signature"
errors since they are incompatible with Gemini API.

Changes:
- Remove fallback to client-provided signatures in thinking block handling
- Only use cached signatures (from same-session Gemini responses)
- Skip thinking blocks without valid cached signatures
- tool_use blocks continue to use skip_thought_signature_validator when
  no valid signature is available

This ensures cross-model switching works correctly while preserving
signature validation for same-model conversations.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/translator/antigravity/claude/antigravity_claude_request.go

@@ -123,11 +123,6 @@ func ConvertClaudeRequestToAntigravity(modelName string, inputRawJSON []byte, _
 					if contentTypeResult.Type == gjson.String && contentTypeResult.String() == "thinking" {
 						// Use GetThinkingText to handle wrapped thinking objects
 						thinkingText := util.GetThinkingText(contentResult)
-						signatureResult := contentResult.Get("signature")
-						clientSignature := ""
-						if signatureResult.Exists() && signatureResult.String() != "" {
-							clientSignature = signatureResult.String()
-						}
 
 						// Always try cached signature first (more reliable than client-provided)
 						// Client may send stale or invalid signatures from different sessions
@@ -139,11 +134,11 @@ func ConvertClaudeRequestToAntigravity(modelName string, inputRawJSON []byte, _
 							}
 						}
 
-						// Fallback to client signature only if cache miss and client signature is valid
-						if signature == "" && cache.HasValidSignature(clientSignature) {
-							signature = clientSignature
-							// log.Debugf("Using client-provided signature for thinking block")
-						}
+						// NOTE: We do NOT fallback to client signature anymore.
+						// Client signatures from Claude models are incompatible with Antigravity/Gemini API.
+						// When switching between models (e.g., Claude Opus -> Gemini Flash), the Claude
+						// signatures will cause "Corrupted thought signature" errors.
+						// If we have no cached signature, the thinking block will be skipped below.
 
 						// Store for subsequent tool_use in the same message
 						if cache.HasValidSignature(signature) {