chore: add data model classes for native Java test vector rewrite

[rishav-karanjit]

Issue #, if available:

Description of changes:

First PR in the TestVectors native Java rewrite series. Sets up the build configuration and introduces the core data model classes that replace the Dafny-generated types.

What changed

Dafny → Java mapping

Dafny Source (TestVectors/dafny/DDBEncryption/src/JsonConfig.dfy)	Native Java Class
datatype Record (line 47)	model/Record.java
datatype LargeRecord line 52	model/LargeRecord.java
datatype TableConfig line 57	model/TableConfig.java
datatype SimpleQuery line 63	model/SimpleQuery.java
datatype ComplexQuery line 70	model/ComplexQuery.java
datatype ComplexTest line 76	model/ComplexTest.java
datatype IoTest line 98	model/IoTest.java
datatype RoundTripTest line 82	model/RoundTripTest.java
datatype WriteTest line 87	model/WriteTest.java
datatype DecryptTest line 92	model/DecryptTest.java
datatype TestVectorConfig (in TestVectors.dfy line 53)	model/TestVectorConfig.java
type ConfigPair = (ConfigName, ConfigName) (line 111)	model/ConfigPair.java

Key type translations

Dafny Type	Java Type
DDB.AttributeMap	Map<String, AttributeValue>
Option<T>	Nullable field (Java 8 compat)
seq<T>	List<T>
map<K, V>	Map<K, V>
ConfigPair = (string, string)	String[] (length 2)
DDB.CreateTableInput	CreateTableRequest
Types.DynamoDbTableEncryptionConfig	DynamoDbTableEncryptionConfig (native DBESDK)

Testing

• gradle clean compileJava passes with no errors

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