Optimize API clients and Jetty runtime for Virtual Threads and high throughput

[ludoch]

This pull request modernizes thread handling across API clients and the Jetty runtime, adding support and protections for Java 21+ Virtual Threads:

1. Bounded Concurrency for Virtual Threads in JdkHttpApiHostClient:

   • When appengine.api.use.virtualthreads is enabled, requests execute on virtual threads, but in-flight HTTP calls to the API host are capped using a Semaphore bounded by maxThreads (APPENGINE_API_MAX_THREADS / APPENGINE_API_MAX_CONNECTIONS, default 100).
   • This prevents unconstrained concurrency bursts and retry storms from saturating the Appserver/Datastore proxy while benefiting from virtual threads.
   • Preserves JDK 17 compilation and execution compatibility via reflective lookup of Executors.newVirtualThreadPerTaskExecutor().

2. Bounded Thread Pools in JettyHttpApiHostClient & JdkHttpApiHostClient:

   • Caps thread pools at maxThreads (default 100) instead of unbounded/200, safely queueing requests during transient backend degradation to avoid triggering masked INTERNAL_ERROR ("Internal Datastore Error") responses.

3. Dynamic JVM Runtime Capability Checks:

   • Updates JettyServletEngineAdapter (Jetty 12 and 12.1) and JettyContainerService (local devappserver) to dynamically check Runtime.version().feature() >= 21 when activating virtual thread pools, ensuring forward compatibility with Java 25+ without hardcoded runtime string dependencies.

4. VirtualThreadSupport Utility Classes:

   • Introduces VirtualThreadSupport in api, api_dev, and runtime/impl using MethodHandles to dynamically detect virtual thread capability and create unstarted virtual threads on JDK 21+ while remaining fully compatible with Java 17 compile targets.

5. Datastore Transaction Backoff Retries:

   • Implements exponential backoff retries in DatastoreServiceImpl.beginTransaction() controlled by appengine.datastore.retries (default 1).

6. Documentation:

   • Adds runtime/runtime_impl_jetty121/API_CLIENTS.md detailing all configuration properties, thread pool bounds, virtual thread behavior, and Datastore retry settings.