test: add unit tests for channel platform matching, interval validation, credits check

- TestIsPlatformPricingMatch: 12 cases covering all platform combinations
- TestMatchingPlatforms: 4 cases for platform expansion
- TestGetChannelModelPricing_AntigravityCrossPlatform: antigravity sees anthropic pricing
- TestGetChannelModelPricing_AnthropicCannotSeeAntigravityPricing: no reverse leakage
- TestResolveChannelMapping_AntigravityCrossPlatform: antigravity uses anthropic mapping
- TestFilterValidIntervals: 8 cases for empty interval filtering
- TestHasEnoughCredits: 10 cases for credits balance threshold logic
- Extract hasEnoughCredits() pure function for testability

backend/internal/service/model_pricing_resolver_test.go

@@ -585,3 +585,79 @@ func TestGetRequestTierPriceByContext_ExactBoundary(t *testing.T) {
 	price2 := r.GetRequestTierPriceByContext(resolved, 128001)
 	require.InDelta(t, 0.10, price2, 1e-12)
 }
+
+// ===========================================================================
+// 8. filterValidIntervals
+// ===========================================================================
+
+func TestFilterValidIntervals(t *testing.T) {
+	tests := []struct {
+		name      string
+		intervals []PricingInterval
+		wantLen   int
+	}{
+		{
+			name:      "empty list",
+			intervals: nil,
+			wantLen:   0,
+		},
+		{
+			name: "all-nil interval filtered out",
+			intervals: []PricingInterval{
+				{MinTokens: 0, MaxTokens: testPtrInt(128000)},
+			},
+			wantLen: 0,
+		},
+		{
+			name: "interval with only InputPrice kept",
+			intervals: []PricingInterval{
+				{MinTokens: 0, MaxTokens: testPtrInt(128000), InputPrice: testPtrFloat64(1e-6)},
+			},
+			wantLen: 1,
+		},
+		{
+			name: "interval with only OutputPrice kept",
+			intervals: []PricingInterval{
+				{MinTokens: 0, MaxTokens: testPtrInt(128000), OutputPrice: testPtrFloat64(2e-6)},
+			},
+			wantLen: 1,
+		},
+		{
+			name: "interval with only CacheWritePrice kept",
+			intervals: []PricingInterval{
+				{MinTokens: 0, CacheWritePrice: testPtrFloat64(3e-6)},
+			},
+			wantLen: 1,
+		},
+		{
+			name: "interval with only CacheReadPrice kept",
+			intervals: []PricingInterval{
+				{MinTokens: 0, CacheReadPrice: testPtrFloat64(0.5e-6)},
+			},
+			wantLen: 1,
+		},
+		{
+			name: "interval with only PerRequestPrice kept",
+			intervals: []PricingInterval{
+				{TierLabel: "1K", PerRequestPrice: testPtrFloat64(0.04)},
+			},
+			wantLen: 1,
+		},
+		{
+			name: "mixed valid and invalid",
+			intervals: []PricingInterval{
+				{MinTokens: 0, MaxTokens: testPtrInt(128000), InputPrice: testPtrFloat64(1e-6)},
+				{MinTokens: 128000, MaxTokens: nil}, // all-nil → filtered out
+				{MinTokens: 256000, OutputPrice: testPtrFloat64(5e-6)},
+			},
+			wantLen: 2,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := filterValidIntervals(tt.intervals)
+			require.Len(t, result, tt.wantLen)
+		})
+	}
+}