docs(java): update event and logger guidance

[zhongkechen]

Summary

• replace the hand-written Lambda event serializer adapter with the optional Java extra-serdes module
• document the getLogger(Logger delegate) overload and clarify that Java has no persistent context-level logger setter

The stale Java 1.2.1 dependency versions and Java 1.x MDC keys were already corrected by #262 and #264.

Dependencies and related changes

• merge after feat: add extra SerDes module and cloud E2E coverage aws-durable-execution-sdk-java#684, which adds the extra-serdes module
• related Java SDK documentation update: docs(logging): document Java 2.x MDC keys aws-durable-execution-sdk-java#692

Validation

• mdformat --check docs/
• codespell docs/sdk-reference/observability/logging.md docs/sdk-reference/languages/java/index.md
• python3 scripts/check_example_refs.py
• git diff --check

The local Zensical build was not run because the pinned zensical==0.0.40 package was unavailable from the configured package index; repository CI remains the authoritative build check.