Address PR review: optional billing fields

Honor the "all billing fields optional" contract surfaced in review:

- Python: deserialize present-but-empty `tokenPrices`/`longContext` objects
  (`{}`) into instances with all fields None rather than collapsing them to
  None, using presence (`is not None`) checks instead of truthiness.
- Java: change `ModelBilling.multiplier` from primitive `double` to boxed
  `Double` so "not present on the wire" is distinguishable from an explicit
  0.0; the global NON_NULL ObjectMapper omits it when null.

Add tests covering both behaviors.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: MackinnonBuck

java/src/main/java/com/github/copilot/rpc/ModelBilling.java

@@ -16,16 +16,16 @@
 public class ModelBilling {
 
     @JsonProperty("multiplier")
-    private double multiplier;
+    private Double multiplier;
 
     @JsonProperty("tokenPrices")
     private ModelBillingTokenPrices tokenPrices;
 
-    public double getMultiplier() {
+    public Double getMultiplier() {
         return multiplier;
     }
 
-    public ModelBilling setMultiplier(double multiplier) {
+    public ModelBilling setMultiplier(Double multiplier) {
         this.multiplier = multiplier;
         return this;
     }

java/src/test/java/com/github/copilot/MetadataApiTest.java

@@ -186,7 +186,7 @@ void testModelInfoDeserialization() throws Exception {
 
         // Billing
         assertNotNull(model.getBilling());
-        assertEquals(1.5, model.getBilling().getMultiplier());
+        assertEquals(Double.valueOf(1.5), model.getBilling().getMultiplier());
 
         // Token prices
         ModelBillingTokenPrices tokenPrices = model.getBilling().getTokenPrices();
@@ -206,6 +206,15 @@ void testModelInfoDeserialization() throws Exception {
         assertEquals(1000000, longContext.getContextMax());
     }
 
+    @Test
+    void testModelBillingSerializationOmitsNullMultiplier() throws Exception {
+        var billing = new ModelBilling();
+
+        String json = MAPPER.writeValueAsString(billing);
+
+        assertFalse(json.contains("multiplier"));
+    }
+
     @Test
     void testGetModelsResponseDeserialization() throws Exception {
         String json = """

python/copilot/client.py

@@ -737,7 +737,7 @@ def from_dict(obj: Any) -> ModelBillingTokenPrices:
         long_context_dict = obj.get("longContext")
         long_context = (
             ModelBillingTokenPricesLongContext.from_dict(long_context_dict)
-            if long_context_dict
+            if long_context_dict is not None
             else None
         )
         return ModelBillingTokenPrices(
@@ -779,7 +779,9 @@ def from_dict(obj: Any) -> ModelBilling:
         multiplier = obj.get("multiplier")
         token_prices_dict = obj.get("tokenPrices")
         token_prices = (
-            ModelBillingTokenPrices.from_dict(token_prices_dict) if token_prices_dict else None
+            ModelBillingTokenPrices.from_dict(token_prices_dict)
+            if token_prices_dict is not None
+            else None
         )
         return ModelBilling(
             multiplier=float(multiplier) if multiplier is not None else None,

python/test_client.py

@@ -552,6 +552,34 @@ def test_token_prices_absent(self):
         assert billing.token_prices is None
         assert billing.to_dict() == {"multiplier": 1.0}
 
+    def test_token_prices_empty_object_round_trip(self):
+        """ModelBilling preserves present but empty tokenPrices."""
+        billing = ModelBilling.from_dict({"tokenPrices": {}})
+
+        assert isinstance(billing.token_prices, ModelBillingTokenPrices)
+        prices = billing.token_prices
+        assert prices.input_price is None
+        assert prices.output_price is None
+        assert prices.cache_price is None
+        assert prices.batch_size is None
+        assert prices.context_max is None
+        assert prices.long_context is None
+        assert billing.to_dict() == {"tokenPrices": {}}
+
+    def test_long_context_empty_object_round_trip(self):
+        """ModelBilling preserves present but empty longContext."""
+        billing = ModelBilling.from_dict({"tokenPrices": {"longContext": {}}})
+
+        assert isinstance(billing.token_prices, ModelBillingTokenPrices)
+        prices = billing.token_prices
+        assert isinstance(prices.long_context, ModelBillingTokenPricesLongContext)
+        long_context = prices.long_context
+        assert long_context.input_price is None
+        assert long_context.output_price is None
+        assert long_context.cache_price is None
+        assert long_context.context_max is None
+        assert billing.to_dict() == {"tokenPrices": {"longContext": {}}}
+
 
 class TestOnListModels:
     @pytest.mark.asyncio