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/gateway_request_test.go

@@ -151,3 +151,125 @@ func TestFilterThinkingBlocks(t *testing.T) {
 		})
 	}
 }
+
+func TestFilterThinkingBlocksForRetry_DisablesThinkingAndPreservesAsText(t *testing.T) {
+	input := []byte(`{
+		"model":"claude-3-5-sonnet-20241022",
+		"thinking":{"type":"enabled","budget_tokens":1024},
+		"messages":[
+			{"role":"user","content":[{"type":"text","text":"Hi"}]},
+			{"role":"assistant","content":[
+				{"type":"thinking","thinking":"Let me think...","signature":"bad_sig"},
+				{"type":"text","text":"Answer"}
+			]}
+		]
+	}`)
+
+	out := FilterThinkingBlocksForRetry(input)
+
+	var req map[string]any
+	require.NoError(t, json.Unmarshal(out, &req))
+	_, hasThinking := req["thinking"]
+	require.False(t, hasThinking)
+
+	msgs, ok := req["messages"].([]any)
+	require.True(t, ok)
+	require.Len(t, msgs, 2)
+
+	assistant := msgs[1].(map[string]any)
+	content := assistant["content"].([]any)
+	require.Len(t, content, 2)
+
+	first := content[0].(map[string]any)
+	require.Equal(t, "text", first["type"])
+	require.Equal(t, "Let me think...", first["text"])
+}
+
+func TestFilterThinkingBlocksForRetry_DisablesThinkingEvenWithoutThinkingBlocks(t *testing.T) {
+	input := []byte(`{
+		"model":"claude-3-5-sonnet-20241022",
+		"thinking":{"type":"enabled","budget_tokens":1024},
+		"messages":[
+			{"role":"user","content":[{"type":"text","text":"Hi"}]},
+			{"role":"assistant","content":[{"type":"text","text":"Prefill"}]}
+		]
+	}`)
+
+	out := FilterThinkingBlocksForRetry(input)
+
+	var req map[string]any
+	require.NoError(t, json.Unmarshal(out, &req))
+	_, hasThinking := req["thinking"]
+	require.False(t, hasThinking)
+}
+
+func TestFilterThinkingBlocksForRetry_RemovesRedactedThinkingAndKeepsValidContent(t *testing.T) {
+	input := []byte(`{
+		"thinking":{"type":"enabled","budget_tokens":1024},
+		"messages":[
+			{"role":"assistant","content":[
+				{"type":"redacted_thinking","data":"..."},
+				{"type":"text","text":"Visible"}
+			]}
+		]
+	}`)
+
+	out := FilterThinkingBlocksForRetry(input)
+
+	var req map[string]any
+	require.NoError(t, json.Unmarshal(out, &req))
+	_, hasThinking := req["thinking"]
+	require.False(t, hasThinking)
+
+	msgs := req["messages"].([]any)
+	content := msgs[0].(map[string]any)["content"].([]any)
+	require.Len(t, content, 1)
+	require.Equal(t, "text", content[0].(map[string]any)["type"])
+	require.Equal(t, "Visible", content[0].(map[string]any)["text"])
+}
+
+func TestFilterThinkingBlocksForRetry_EmptyContentGetsPlaceholder(t *testing.T) {
+	input := []byte(`{
+		"thinking":{"type":"enabled"},
+		"messages":[
+			{"role":"assistant","content":[{"type":"redacted_thinking","data":"..."}]}
+		]
+	}`)
+
+	out := FilterThinkingBlocksForRetry(input)
+
+	var req map[string]any
+	require.NoError(t, json.Unmarshal(out, &req))
+	msgs := req["messages"].([]any)
+	content := msgs[0].(map[string]any)["content"].([]any)
+	require.Len(t, content, 1)
+	require.Equal(t, "text", content[0].(map[string]any)["type"])
+	require.NotEmpty(t, content[0].(map[string]any)["text"])
+}
+
+func TestFilterSignatureSensitiveBlocksForRetry_DowngradesTools(t *testing.T) {
+	input := []byte(`{
+		"thinking":{"type":"enabled","budget_tokens":1024},
+		"messages":[
+			{"role":"assistant","content":[
+				{"type":"tool_use","id":"t1","name":"Bash","input":{"command":"ls"}},
+				{"type":"tool_result","tool_use_id":"t1","content":"ok","is_error":false}
+			]}
+		]
+	}`)
+
+	out := FilterSignatureSensitiveBlocksForRetry(input)
+
+	var req map[string]any
+	require.NoError(t, json.Unmarshal(out, &req))
+	_, hasThinking := req["thinking"]
+	require.False(t, hasThinking)
+
+	msgs := req["messages"].([]any)
+	content := msgs[0].(map[string]any)["content"].([]any)
+	require.Len(t, content, 2)
+	require.Equal(t, "text", content[0].(map[string]any)["type"])
+	require.Equal(t, "text", content[1].(map[string]any)["type"])
+	require.Contains(t, content[0].(map[string]any)["text"], "tool_use")
+	require.Contains(t, content[1].(map[string]any)["text"], "tool_result")
+}