test: add concurrent access functional tests to Java bindings

[aepfli]

Problem

The Java FlagEvaluator uses synchronized methods because the WASM runtime is single-threaded. While PR #64 added JMH concurrent benchmarks, there are no functional correctness tests that verify thread safety under concurrent access.

Concurrent access is a common real-world pattern — multiple threads evaluating flags simultaneously while another thread updates state.

Scope

Add JUnit tests that verify:

1. Concurrent evaluation — Multiple threads evaluating different flags simultaneously; all get correct results
2. Concurrent evaluate + updateState — One thread updating flag state while others are evaluating; no crashes or corrupted results
3. State consistency — After updateState, all subsequent evaluations (from any thread) see the new state
4. No deadlocks — High-contention scenarios with many threads don't deadlock

Implementation Notes

• Use ExecutorService with a thread pool (e.g., 8-16 threads)
• Use CountDownLatch for coordinated start
• Run enough iterations (1000+) to surface race conditions
• Assert correctness of every evaluation result, not just absence of exceptions

Reference

• FlagEvaluator synchronization: java/src/main/java/dev/openfeature/flagd/evaluator/FlagEvaluator.java
• JMH benchmarks (PR bench(java): add multithreaded JMH benchmarks for concurrent evaluation #64): java/src/main/java/dev/openfeature/flagd/evaluator/FlagEvaluatorBenchmark.java
• Part of the test gap analysis from the cross-implementation review