docs(java): document wait strategy helpers

[zhongkechen]

Summary

• document WaitStrategies.defaultStrategy() and Presets.DEFAULT
• add Java examples for exponential-backoff and fixed-delay strategies
• explain the exact Java defaults, one-based attempt counting, and max-attempt failure behavior
• clarify that Java completion decisions belong to the check function rather than the delay strategy

Testing

• /tmp/durable-docs-should-complete-venv/bin/mdformat --check docs/
• /tmp/durable-docs-should-complete-venv/bin/codespell docs/ examples/java/core/wait/strategy-helper.java examples/java/core/wait/wait-for-condition-signature.java
• python3.14 scripts/check_example_refs.py
• python3.14 scripts/vendor_mermaid.py --check
• python3.14 scripts/vendor_glightbox.py --check
• /tmp/durable-docs-should-complete-py314/bin/zensical build --clean
• hidden Unicode check
• verified all documented APIs against Java main