feat(antigravity): configurable signature cache with bypass-mode validation

Antigravity 的 Claude thinking signature 处理新增 cache/bypass 双模式，
并为 bypass 模式实现按 SIGNATURE-CHANNEL-SPEC.md 的签名校验。

新增 antigravity-signature-cache-enabled 配置项（默认 true）：
- cache mode（true）：使用服务端缓存的签名，行为与原有逻辑完全一致
- bypass mode（false）：直接使用客户端提供的签名，经过校验和归一化

支持配置热重载，运行时可切换模式。

校验流程：
1. 剥离历史 cache-mode 的 'modelGroup#' 前缀（如 claude#Exxxx → Exxxx）
2. 首字符必须为 'E'（单层编码）或 'R'（双层编码），否则拒绝
3. R 开头：base64 解码 → 内层必须以 'E' 开头 → 继续单层校验
4. E 开头：base64 解码 → 首字节必须为 0x12（Claude protobuf 标识）
5. 所有合法签名归一化为 R 形式（双层 base64）发往 Antigravity 后端

非法签名处理策略：
- 非严格模式（默认）：translator 静默丢弃无签名的 thinking block
- 严格模式（antigravity-signature-bypass-strict: true）：
  executor 层在请求发往上游前直接返回 HTTP 400

按 SIGNATURE-CHANNEL-SPEC.md 解析 Claude 签名的完整 protobuf 结构：
- Top-level Field 2（容器）→ Field 1（渠道块）
- 渠道块提取：channel_id (Field 1)、infrastructure (Field 2)、
  model_text (Field 6)、field7 (Field 7)
- 计算 routing_class、infrastructure_class、schema_features
- 使用 google.golang.org/protobuf/encoding/protowire 解析

- resolveThinkingSignature 拆分为 resolveCacheModeSignature / resolveBypassModeSignature
- hasResolvedThinkingSignature：mode-aware 签名有效性判断
  （cache: len>=50 via HasValidSignature，bypass: non-empty）
- validateAntigravityRequestSignatures：executor 预检，
  仅在 bypass + strict 模式下拦截非法签名返回 400
- 响应侧签名缓存逻辑与 cache mode 集成
- Cache mode 行为完全保留：无 '#' 前缀的原生签名静默丢弃

internal/translator/antigravity/claude/antigravity_claude_response_test.go

@@ -1,6 +1,7 @@
 package claude
 
 import (
+	"bytes"
 	"context"
 	"strings"
 	"testing"
@@ -244,3 +245,105 @@ func TestConvertAntigravityResponseToClaude_MultipleThinkingBlocks(t *testing.T)
 		t.Error("Second thinking block signature should be cached")
 	}
 }
+
+func TestConvertAntigravityResponseToClaude_TextAndSignatureInSameChunk(t *testing.T) {
+	cache.ClearSignatureCache("")
+
+	requestJSON := []byte(`{
+		"model": "claude-sonnet-4-5-thinking",
+		"messages": [{"role": "user", "content": [{"type": "text", "text": "Test"}]}]
+	}`)
+
+	validSignature := "RtestSig1234567890123456789012345678901234567890123456789"
+
+	// Chunk 1: thinking text only (no signature)
+	chunk1 := []byte(`{
+		"response": {
+			"candidates": [{
+				"content": {
+					"parts": [{"text": "First part.", "thought": true}]
+				}
+			}]
+		}
+	}`)
+
+	// Chunk 2: thinking text AND signature in the same part
+	chunk2 := []byte(`{
+		"response": {
+			"candidates": [{
+				"content": {
+					"parts": [{"text": " Second part.", "thought": true, "thoughtSignature": "` + validSignature + `"}]
+				}
+			}]
+		}
+	}`)
+
+	var param any
+	ctx := context.Background()
+
+	result1 := ConvertAntigravityResponseToClaude(ctx, "claude-sonnet-4-5-thinking", requestJSON, requestJSON, chunk1, &param)
+	result2 := ConvertAntigravityResponseToClaude(ctx, "claude-sonnet-4-5-thinking", requestJSON, requestJSON, chunk2, &param)
+
+	allOutput := string(bytes.Join(result1, nil)) + string(bytes.Join(result2, nil))
+
+	// The text " Second part." must appear as a thinking_delta, not be silently dropped
+	if !strings.Contains(allOutput, "Second part.") {
+		t.Error("Text co-located with signature must be emitted as thinking_delta before the signature")
+	}
+
+	// The signature must also be emitted
+	if !strings.Contains(allOutput, "signature_delta") {
+		t.Error("Signature delta must still be emitted")
+	}
+
+	// Verify the cached signature covers the FULL text (both parts)
+	fullText := "First part. Second part."
+	cachedSig := cache.GetCachedSignature("claude-sonnet-4-5-thinking", fullText)
+	if cachedSig != validSignature {
+		t.Errorf("Cached signature should cover full text %q, got sig=%q", fullText, cachedSig)
+	}
+}
+
+func TestConvertAntigravityResponseToClaude_SignatureOnlyChunk(t *testing.T) {
+	cache.ClearSignatureCache("")
+
+	requestJSON := []byte(`{
+		"model": "claude-sonnet-4-5-thinking",
+		"messages": [{"role": "user", "content": [{"type": "text", "text": "Test"}]}]
+	}`)
+
+	validSignature := "RtestSig1234567890123456789012345678901234567890123456789"
+
+	// Chunk 1: thinking text
+	chunk1 := []byte(`{
+		"response": {
+			"candidates": [{
+				"content": {
+					"parts": [{"text": "Full thinking text.", "thought": true}]
+				}
+			}]
+		}
+	}`)
+
+	// Chunk 2: signature only (empty text) — the normal case
+	chunk2 := []byte(`{
+		"response": {
+			"candidates": [{
+				"content": {
+					"parts": [{"text": "", "thought": true, "thoughtSignature": "` + validSignature + `"}]
+				}
+			}]
+		}
+	}`)
+
+	var param any
+	ctx := context.Background()
+
+	ConvertAntigravityResponseToClaude(ctx, "claude-sonnet-4-5-thinking", requestJSON, requestJSON, chunk1, &param)
+	ConvertAntigravityResponseToClaude(ctx, "claude-sonnet-4-5-thinking", requestJSON, requestJSON, chunk2, &param)
+
+	cachedSig := cache.GetCachedSignature("claude-sonnet-4-5-thinking", "Full thinking text.")
+	if cachedSig != validSignature {
+		t.Errorf("Signature-only chunk should still cache correctly, got %q", cachedSig)
+	}
+}