fix(service): normalize user agent for gemini session reuse

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent

)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/internal/service/gemini_session.go

@@ -52,10 +52,11 @@ func BuildGeminiDigestChain(req *antigravity.GeminiRequest) string {
 // 返回 16 字符的 Base64 编码的 SHA256 前缀
 func GenerateGeminiPrefixHash(userID, apiKeyID int64, ip, userAgent, platform, model string) string {
 	// 组合所有标识符
+	normalizedUserAgent := NormalizeSessionUserAgent(userAgent)
 	combined := strconv.FormatInt(userID, 10) + ":" +
 		strconv.FormatInt(apiKeyID, 10) + ":" +
 		ip + ":" +
-		userAgent + ":" +
+		normalizedUserAgent + ":" +
 		platform + ":" +
 		model
 

backend/internal/service/gemini_session_test.go

@@ -152,6 +152,24 @@ func TestGenerateGeminiPrefixHash(t *testing.T) {
 	}
 }
 
+func TestGenerateGeminiPrefixHash_IgnoresUserAgentVersionNoise(t *testing.T) {
+	hash1 := GenerateGeminiPrefixHash(1, 100, "192.168.1.1", "Mozilla/5.0 codex_cli_rs/0.1.0", "antigravity", "gemini-2.5-pro")
+	hash2 := GenerateGeminiPrefixHash(1, 100, "192.168.1.1", "Mozilla/5.0 codex_cli_rs/0.1.1", "antigravity", "gemini-2.5-pro")
+
+	if hash1 != hash2 {
+		t.Fatalf("version-only User-Agent changes should not perturb Gemini prefix hash: %s vs %s", hash1, hash2)
+	}
+}
+
+func TestGenerateGeminiPrefixHash_IgnoresFreeformUserAgentVersionNoise(t *testing.T) {
+	hash1 := GenerateGeminiPrefixHash(1, 100, "192.168.1.1", "Codex CLI 0.1.0", "antigravity", "gemini-2.5-pro")
+	hash2 := GenerateGeminiPrefixHash(1, 100, "192.168.1.1", "Codex CLI 0.1.1", "antigravity", "gemini-2.5-pro")
+
+	if hash1 != hash2 {
+		t.Fatalf("free-form version-only User-Agent changes should not perturb Gemini prefix hash: %s vs %s", hash1, hash2)
+	}
+}
+
 func TestParseGeminiSessionValue(t *testing.T) {
 	tests := []struct {
 		name      string