feat(translator): normalize service_tier in Codex requests and add tests

- Added `normalizeCodexServiceTier` to standardize `service_tier` values (`fast` to `priority`, omit unsupported tiers).
- Updated `ConvertClaudeRequestToCodex` to apply normalization logic.
- Introduced `TestConvertClaudeRequestToCodex_ServiceTier` to validate behavior across various `service_tier` inputs.

Closes: #3276 #3294

internal/translator/codex/claude/codex_claude_request.go

@@ -327,6 +327,9 @@ func ConvertClaudeRequestToCodex(modelName string, inputRawJSON []byte, _ bool)
 	}
 	template, _ = sjson.SetBytes(template, "reasoning.effort", reasoningEffort)
 	template, _ = sjson.SetBytes(template, "reasoning.summary", "auto")
+	if serviceTier := normalizeCodexServiceTier(rootResult.Get("service_tier")); serviceTier != "" {
+		template, _ = sjson.SetBytes(template, "service_tier", serviceTier)
+	}
 	template, _ = sjson.SetBytes(template, "stream", true)
 	template, _ = sjson.SetBytes(template, "store", false)
 	template, _ = sjson.SetBytes(template, "include", []string{"reasoning.encrypted_content"})
@@ -334,6 +337,19 @@ func ConvertClaudeRequestToCodex(modelName string, inputRawJSON []byte, _ bool)
 	return template
 }
 
+func normalizeCodexServiceTier(result gjson.Result) string {
+	if !result.Exists() || result.Type != gjson.String {
+		return ""
+	}
+
+	switch strings.ToLower(strings.TrimSpace(result.String())) {
+	case "fast", "priority":
+		return "priority"
+	default:
+		return ""
+	}
+}
+
 // shortenCodexCallIDIfNeeded keeps Claude tool IDs within the OpenAI Responses
 // API call_id limit while preserving a stable, low-collision mapping.
 func shortenCodexCallIDIfNeeded(id string) string {

internal/translator/codex/claude/codex_claude_request_test.go

@@ -148,6 +148,58 @@ func TestConvertClaudeRequestToCodex_ParallelToolCalls(t *testing.T) {
 	}
 }
 
+func TestConvertClaudeRequestToCodex_ServiceTier(t *testing.T) {
+	tests := []struct {
+		name            string
+		serviceTierJSON string
+		want            string
+		wantExists      bool
+	}{
+		{
+			name:            "Priority passes through",
+			serviceTierJSON: `"priority"`,
+			want:            "priority",
+			wantExists:      true,
+		},
+		{
+			name:            "Fast normalizes to priority",
+			serviceTierJSON: `"fast"`,
+			want:            "priority",
+			wantExists:      true,
+		},
+		{
+			name:            "Unsupported tier is omitted",
+			serviceTierJSON: `"default"`,
+		},
+		{
+			name:            "Non-string tier is omitted",
+			serviceTierJSON: `true`,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			inputJSON := `{
+				"model": "gpt-5.4",
+				"service_tier": ` + tt.serviceTierJSON + `,
+				"messages": [{"role": "user", "content": "Reply with OK"}]
+			}`
+
+			result := ConvertClaudeRequestToCodex("gpt-5.4", []byte(inputJSON), false)
+			serviceTierResult := gjson.GetBytes(result, "service_tier")
+			if serviceTierResult.Exists() != tt.wantExists {
+				t.Fatalf("service_tier exists = %v, want %v. Output: %s", serviceTierResult.Exists(), tt.wantExists, string(result))
+			}
+			if !tt.wantExists {
+				return
+			}
+			if got := serviceTierResult.String(); got != tt.want {
+				t.Fatalf("service_tier = %q, want %q. Output: %s", got, tt.want, string(result))
+			}
+		})
+	}
+}
+
 func TestConvertClaudeRequestToCodex_ShortenLongToolUseIDs(t *testing.T) {
 	longID := "toolu_" + strings.Repeat("a", 62)
 	if len(longID) <= 64 {