Fix DataContractSerializer nullable struct initialization cache lookup

[Copilot]

Description

DataContractSerializer regressed in .NET 8+ when deserializing Nullable<T> for [DataContract] structs that had not been initialized in the cache path yet, causing SerializationException (DataContract cache overflow).
The initialization lookup loop compared against the wrong cache slot, so valid contracts were not found.

• Runtime fix (System.Private.DataContractSerialization)

  Corrected DataContract.GetIdForInitialization(ClassDataContract) to scan cache entries by loop index instead of repeatedly reading the originally computed id.

  for (int i = 0; i < currentDataContractId; i++)
  {
      if (ContractMatches(classContract, s_dataContractCache.GetItem(i)))
      {
          return i;
      }
  }

• Regression coverage (System.Runtime.Serialization.Xml tests)

  Added a focused test that round-trips a nullable root [DataContract] struct and verifies successful deserialization without requiring prior serialization of the non-nullable struct type.