[dajiaohuang]

Summary

Fixes #430. The main and beta async message services composed prepareAsync() with thenComposeAsync() at each of their three request call sites. With the current completed request-preparation future, that schedules HttpClient.executeAsync() on ForkJoinPool.commonPool() and drops caller thread-local context before transport instrumentation observes the request.

Use non-async thenCompose() for create, streaming create, and count-tokens requests in both services. This avoids the implicit common-pool hop and preserves the preparation/completion thread's context; streamHandlerExecutor remains dedicated to stream callbacks.

Validation

• Added six synthetic tests using a fake HttpClient and a ThreadLocal; they cover all three methods in both main and beta services.
• On frozen snapshot base afb2c556f90c2da15f5ebd63ee7be604cf59edaf, the two streaming regressions failed before the fix (context was null) and all six tests passed after it.
• Rebasing onto current main (c0f5915f7dd34cef8d5f03fdc8cdd4f93933c47e) succeeded. A post-rebase focused Gradle run timed out after 600 seconds during :anthropic-java-core:compileTestKotlin; no test result was produced by that attempt. No provider API or external test endpoint was used.