…ation

…ture documentation

…tion and enhance concurrency management

Covers all non-trivial logic with 61 tests across 10 test classes:
- AccountMigrationJobTest: entity state machine (PENDING→RUNNING→COMPLETED/FAILED/CANCELLED)
- CancellationTokenTest: thread-safe cancel/isCancelled including virtual-thread visibility
- KeepIdsIdentityMapperTest / RegenerateIdsIdentityMapperTest: both identity strategies
- EntityDependencyGraphTest: Kahn's BFS topological sort with Mockito JPA metamodel stubs
- GzipStreamTest: GZIP magic-byte detection and BufferedInputStream mark() contract (P1 fix coverage)
- ExportConstantsTest: JSON format constants and minimal valid stream structure
- ImportPipelineMapperResolutionTest: UUID7 guard, custom Spring bean mapper priority
- OptionsFluentBuilderTest: fluent builders and Jackson round-trip for options_json
- AccountMigrationPropertiesTest: defaults and semaphore floor guard

Adds mockito-core:5.20.0 as test-scope dependency.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Spring Boot 4 ships Jackson 3 under the tools.jackson.* groupId.
Updated pom.xml, all production sources, and test files to use the new
packages and renamed API surface:
- jackson-databind groupId: com.fasterxml.jackson.core → tools.jackson.core
- Removed jackson-datatype-jsr310 (Java Time support is built-in to databind 3.x)
- ObjectMapper: getFactory().createGenerator/Parser → createGenerator/createParser
- JsonGenerator: writeFieldName → writeName, writeObject → writePOJO,
  writeXxxField → writeXxxProperty, writeObjectFieldStart/writeArrayFieldStart
  → writeName + writeStartObject/writeStartArray
- JsonMapper.builder(): removed .disable(WRITE_DATES_AS_TIMESTAMPS) (ISO-8601
  is the default in Jackson 3, no flag needed)
- Test setUp(): removed .addModule(new JavaTimeModule()) (module no longer exists)

All 61 unit tests pass.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>