Add checkstyle rules fit for v2

[RanVaknin]

mapper v2 house cleaning PR

dynamodb-mapper (mapper v2) is a faithful 1:1 port of the EOL v1 DynamoDBMapper.
Enabling the full v2 checkstyle ruleset would rename public types, change signatures,
reorder members, and add code absent from v1. That turns the port into a refactor: harder
to diff against v1, and a risk to behavior.

This PR is house-cleaning to fit the module into the v2 repo without touching runtime
behavior. Every commit is either mechanical or low-discretion; the split below marks which.

Commits

1. Checkstyle allowlist + SDK annotations (mechanical, some discretion)

Instead of the full ruleset we allowlist 3 rules:

• HideUtilityClassConstructor (5): added private constructors to utility classes. Mechanical.
• UnnecessaryFinalOnLocalVariable (216): removed unnecessary final on locals across 17 files. Mechanical.
• MissingSdkAnnotation (110): added SDK API annotations. Discretion: classification follows the Enhanced Client convention.
  • 40 public-visibility types → @SdkPublicApi
  • 68 marshaller/unmarshaller converters + 2 package-private helpers → @SdkInternalApi
  • No @SdkProtectedApi

2. Repo layout (mechanical)

Moved marshallers/unmarshallers/utils under internal/; moved AWS-credentialed integ tests to src/it/. Package-relative moves, no code changes.

3. Swap remaining v1 imports + logging (mechanical)

Remaining v1 SDK imports → v2 equivalents; commons-logging → SLF4J.

4. Remove dead code (mechanical)

Deleted unused DynamoDBReflector/ReflectionUtils; removed deprecated RequestMetricCollector from config (no v2 equivalent).

5. Restore data-plane functional tests (mechanical)

Un-excluded the DynamoDB Local functional tests that only use table admin for fixture setup (data-plane coverage, runs on the raw v2 client). Pure table-admin and S3Link tests stay excluded pending their ports.

6. Replace log4j 1.x with log4j2 (mechanical)

Swapped the log4j 1.x test dep for the SDK-standard log4j-api/log4j-core/log4j-slf4j-impl (bom-managed); replaced log4j.properties with log4j2.xml.

Not in this PR (deferred to dedicated port PRs)

Table-admin/GSI port, S3Link port, and the v1-coupled tests they block. ReflectionUtils stays @Deprecated until then.

---

**DDB mapper v2 roadmap: **

** BASE PACKAGE SETUP **
+ 1. Source verbatim port ✅
+ 2. Test verbatim port ✅
+ 3. Namespace swap (main + datamodeling tests) ✅ 
+ 4. Namespace swap (remaining test packages) ✅

** PORTING OPERATIONS **
+ 0. converters (AttributeValue seam) ✅ 
+ 1. load() ✅
+ 2. save() ✅
+ 3. query() + scan() ✅
+ 4. deleteItem() ✅
+ 5. updateItem() ✅
+ 6. Batch operations ✅
+ 7. Transactions ✅
8. S3Link
9. Table Admin (control plane convenience methods) 
- 10. Cleanup: <--- current PR

** PERFORMANCE IMPROVEMENTS **
1. getTableModel caching
2. Wire "fast" createX AV factory methods to convertors
3. ByteBuffer → SdkBytes copy
4. Others

** DEPENDENCY MODERNIZATION **
+ 1. EasyMock -> Mockito ✅
- 2. Log4j 1.x -> 2.x  <--- current PR
- 3. commons-logging -> SLF4J  <--- current PR

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Appendix: full annotation classification

@SdkPublicApi: 40 public-visibility types

Core mapper (7)
DynamoDBMapper, AbstractDynamoDBMapper, IDynamoDBMapper, DynamoDBMapperConfig,
DynamoDBTableMapper, DynamoDBMapperTableModel, DynamoDBMapperFieldModel

Operation expressions / requests (9)
DynamoDBQueryExpression, DynamoDBScanExpression, DynamoDBSaveExpression, DynamoDBDeleteExpression,
DynamoDBTransactionLoadExpression, DynamoDBTransactionWriteExpression,
TransactionLoadRequest, TransactionWriteRequest, BatchLoadContext

Result / pagination types (6)
PaginatedList, PaginatedQueryList, PaginatedScanList, PaginatedParallelScanList,
QueryResultPage, ScanResultPage

Conversion & extension SPIs (10)
DynamoDBTypeConverter, DynamoDBTypeConverterFactory, ConversionSchema, ConversionSchemas,
ItemConverter, DynamoDBAutoGenerator, AttributeTransformer, AttributeTransformerChain,
ArgumentMarshaller, ArgumentUnmarshaller

S3 / misc (4)
S3Link, S3ClientCache, KeyPair, DynamoDBMappingException

Deprecated but public (4)
DynamoDBMarshaller, AbstractEnumMarshaller, JsonMarshaller, ParallelScanTask

@SdkInternalApi: 70 types

• marshallers/*: 23 converter classes (Java type → AttributeValue)
• unmarshallers/*: 45 converter classes (AttributeValue → Java type)
• MapperExceptions: internal exception helper