fix(backend): 改进 thinking/tool block 签名处理和重试策略

主要改动：
- request_transformer: thinking block 缺少签名时降级为文本而非丢弃，保留内容并在上层禁用 thinking mode
- antigravity_gateway_service: 新增两阶段降级策略，先处理 thinking blocks，如仍失败且涉及 tool 签名错误则进一步降级 tool blocks
- gateway_request: 新增 FilterSignatureSensitiveBlocksForRetry 函数，支持将 tool_use/tool_result 降级为文本
- gateway_request: 改进 FilterThinkingBlocksForRetry，禁用顶层 thinking 配置以避免结构约束冲突
- gateway_service: 实现保守的两阶段重试逻辑，优先保留内容，仅在必要时降级工具调用
- 新增 antigravity_gateway_service_test.go 测试签名块剥离逻辑
- 更新相关测试用例以验证降级行为

此修复解决了跨平台/账户切换时历史消息签名失效导致的请求失败问题。

backend/internal/service/antigravity_gateway_service_test.go

@@ -0,0 +1,83 @@
+package service
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/Wei-Shaw/sub2api/internal/pkg/antigravity"
+	"github.com/stretchr/testify/require"
+)
+
+func TestStripSignatureSensitiveBlocksFromClaudeRequest(t *testing.T) {
+	req := &antigravity.ClaudeRequest{
+		Model: "claude-sonnet-4-5",
+		Thinking: &antigravity.ThinkingConfig{
+			Type:         "enabled",
+			BudgetTokens: 1024,
+		},
+		Messages: []antigravity.ClaudeMessage{
+			{
+				Role:    "assistant",
+				Content: json.RawMessage(`[
+					{"type":"thinking","thinking":"secret plan","signature":""},
+					{"type":"tool_use","id":"t1","name":"Bash","input":{"command":"ls"}}
+				]`),
+			},
+			{
+				Role:    "user",
+				Content: json.RawMessage(`[
+					{"type":"tool_result","tool_use_id":"t1","content":"ok","is_error":false},
+					{"type":"redacted_thinking","data":"..."}
+				]`),
+			},
+		},
+	}
+
+	changed, err := stripSignatureSensitiveBlocksFromClaudeRequest(req)
+	require.NoError(t, err)
+	require.True(t, changed)
+	require.Nil(t, req.Thinking)
+
+	require.Len(t, req.Messages, 2)
+
+	var blocks0 []map[string]any
+	require.NoError(t, json.Unmarshal(req.Messages[0].Content, &blocks0))
+	require.Len(t, blocks0, 2)
+	require.Equal(t, "text", blocks0[0]["type"])
+	require.Equal(t, "secret plan", blocks0[0]["text"])
+	require.Equal(t, "text", blocks0[1]["type"])
+
+	var blocks1 []map[string]any
+	require.NoError(t, json.Unmarshal(req.Messages[1].Content, &blocks1))
+	require.Len(t, blocks1, 1)
+	require.Equal(t, "text", blocks1[0]["type"])
+	require.NotEmpty(t, blocks1[0]["text"])
+}
+
+func TestStripThinkingFromClaudeRequest_DoesNotDowngradeTools(t *testing.T) {
+	req := &antigravity.ClaudeRequest{
+		Model: "claude-sonnet-4-5",
+		Thinking: &antigravity.ThinkingConfig{
+			Type:         "enabled",
+			BudgetTokens: 1024,
+		},
+		Messages: []antigravity.ClaudeMessage{
+			{
+				Role:    "assistant",
+				Content: json.RawMessage(`[{"type":"thinking","thinking":"secret plan"},{"type":"tool_use","id":"t1","name":"Bash","input":{"command":"ls"}}]`),
+			},
+		},
+	}
+
+	changed, err := stripThinkingFromClaudeRequest(req)
+	require.NoError(t, err)
+	require.True(t, changed)
+	require.Nil(t, req.Thinking)
+
+	var blocks []map[string]any
+	require.NoError(t, json.Unmarshal(req.Messages[0].Content, &blocks))
+	require.Len(t, blocks, 2)
+	require.Equal(t, "text", blocks[0]["type"])
+	require.Equal(t, "secret plan", blocks[0]["text"])
+	require.Equal(t, "tool_use", blocks[1]["type"])
+}