feat: migrate to Jackson 3

[GabrielBBaldez]

Purpose

Migrates the SDK from Jackson 2.17.3 to Jackson 3.0.0, as requested in #370.

⚠️ Jackson 3 requires Java 17. This PR therefore bumps the project baseline from Java 8 to Java 17 (build.gradle + CI workflows), which drops Java 8/11 support. That is an inherent consequence of Jackson 3 and a release-level decision — flagging it explicitly.

What changed

• build.gradle: switched to the tools.jackson BOM (jackson-databind 3.0.0). jackson-annotations intentionally stays under com.fasterxml.jackson (Jackson 3 keeps the annotations package).
• Java baseline → 17: sourceCompatibility bumped 8 → 17 and the CI workflows (basic.yml, docs.yml) bumped to Java 17 (Jackson 3's minimum). Removed import lombok.var; (3 files) in favor of native var.
• Imports: com.fasterxml.jackson.{core,databind}.* → tools.jackson.* (annotations untouched; JsonFactory moved to tools.jackson.core.json).
• JacksonJsonTransformer: ObjectMapper is immutable in 3.x, so the mapper is built with JsonMapper.builder() (configure, changeDefaultPropertyInclusion(NON_NULL), defaultDateFormat, addModule, changeDefaultVisibility); derived mappers use rebuild() instead of copy().
• Custom (de)serializers:
  • JsonSerializer/JsonDeserializer → ValueSerializer/ValueDeserializer
  • SerializerProvider → SerializationContext
  • ContextualDeserializer removed — createContextual is now declared on ValueDeserializer
  • p.getCodec().readTree(p) → ctxt.readTree(p); getCurrentToken() → currentToken(); node.fieldNames() → propertyNames()
  • dropped throws IOException (Jackson exceptions are unchecked in 3.x)
• Tests: the deserializer unit tests relied on Jackson 2 internals that were removed in 3.x (JsonParser.setCodec, ObjectMapper.getFactory, ObjectMapper.getDeserializationContext). They were rewritten to drive deserialization through a configured mapper (readValue), preserving the same assertions.

Heads-up (behavioral change)

Jackson 3's StdDateFormat serializes UTC dates as ...Z (canonical ISO-8601) instead of ...+00:00. For example 2019-09-23T11:26:54.000+00:00 → 2019-09-23T11:26:54.000Z. The affected request fixture was updated. There is no built-in option in Jackson 3's StdDateFormat to restore the +00:00 form; both are valid ISO-8601.

Testing

All 579 tests pass locally (./gradlew test, JDK 17).

This is a breaking change (requires Java 17, new tools.jackson coordinates, and the date-format note above), so it's presumably for the next major release — happy to align on timing/approach.

Closes #370

[GabrielBBaldez]

Heads-up on the build failure — it turned out to be the key point of this migration: Jackson 3 requires Java 17, but the project currently compiles with Java 8 (basic.yml) and build.gradle had sourceCompatibility = 8. So the build couldn't even read Jackson 3's class files (error: cannot access tools.jackson.databind.annotation.JsonDeserialize).

I pushed the necessary follow-up:

• bumped sourceCompatibility to 17 and the CI workflows (basic.yml, docs.yml) to Java 17;
• removed import lombok.var; (in HttpException, Sandbox, CrowdinApiTest) — native var (Java 10+) replaces it.

So migrating to Jackson 3 inherently requires Java 17 and drops Java 8/11 support. That's a release-level decision rather than something this PR can avoid — flagging it explicitly so you can decide whether it fits the intended target release. All 579 tests pass locally on JDK 17.