feat(aws-serverless): add Java language coverage to aws-lambda-durable-functions skill

[DeepaliTandale]

Summary

Add comprehensive Java examples to all reference documentation for the Lambda Durable Functions skill, ensuring complete language coverage across TypeScript, Python, and Java.

Changes

Java examples added to 10 files (1,892 insertions, 1 in-place edit):

• SKILL.md — prerequisites, language selection, Maven install, basic handler, Java API section, resources
• getting-started.md — basic handler, multi-step, GenAI agent, approval, saga, project structure, Maven setup, workflow, checklist
• replay-model-rules.md — all 4 rules (wrong + correct), 3 pitfalls, testing replay
• step-operations.md — typed steps, retry, custom retry, selective retry, semantics, serialization, child-context, error handling
• wait-operations.md — simple waits, callback (with SDK callback-success), wait-for-condition, custom strategy, 3 callback patterns, error handling
• concurrent-operations.md — map, parallel, 3 completion policies, result handling, error handling, concurrency control, advanced patterns, performance
• error-handling.md — retry, custom retry, classification, saga, unrecoverable, Java exception table, determinism, partial failure
• advanced-error-handling.md — DurableFuture.anyOf, Java exception types table
• advanced-patterns.md — GenAI agent, step semantics, completion combinations, early termination, serialization, nested workflows, Java SDK configuration (virtual threads + checkpoint delay)
• testing-patterns.md — 12 blocks covering local setup, operations, replay, waits, API patterns, callbacks, errors, concurrency, cloud testing, assertions + Java common-errors table

Verification

• All Java snippets compile against the published aws-durable-execution-sdk-java 1.2.1
• 27 JUnit tests execute via LocalDurableTestRunner (happy path, callbacks, retry, saga, map, parallel, waitForCondition, virtual-thread config, unrecoverable errors, user end-to-end scenario)
• APIs verified against SDK source (DurableContext.java, config/exception/retry/model packages) — not assumptions
• No TypeScript or Python code was modified

Acknowledgment

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of the project license.

[DeepaliTandale]

@bfreiberg @dhegberg This replaces the previously-closed PR #112, rebuilt from scratch on the latest main with all review feedback addressed and every Java snippet compile-verified against SDK 1.2.1.