Fix Vertx cats fs2 streaming deadlock on limited executors

[arimu1]

Summary

• Fixes a deadlock when VertxCatsServerInterpreter streams response bodies via fs2 while the cats-effect Dispatcher runs on a small fixed thread pool (e.g. ZIO with a 2-thread executor).
• Refactors mapToReadStream to return the Vert.x ReadStream immediately using JVM concurrency primitives (AtomicReference, CompletableFuture) for pause/resume and handler wiring, and starts the fs2 fiber with unsafeRunAndForget instead of blocking via unsafeRunSync.

Fixes #5458

Problem

Parallel streaming responses called dispatcher.unsafeRunSync during asReadStream setup. When the dispatcher shares a limited compute pool with request handling (as in the issue repro with ZIO), all pool threads block waiting for effects that need those same threads — freezing the app.

Test plan

• sbt "vertxServerCats3/Test/testOnly sttp.tapir.server.vertx.cats.streams.Fs2StreamTest" (JDK 17) — all 5 tests pass
• Existing coverage exercises pause/resume, interruption, and bidirectional read-stream draining

Made with Cursor

[arimu1]

CI note: the lone red ci (2.13, JVM, 11) job failed resolving org.scala-sbt:sbt:1.12.14 (forbidden from Maven Central / scala-sbt repos) before tests ran — looks like an sbt launcher/infra issue rather than this change. Other matrix cells were green. Happy to re-trigger when sbt 1.12.14 is fetchable again.