Add UndecoratedRunnable to skip onSchedule decoration

[hocaron]

What

Adds UndecoratedRunnable, a marker interface for Runnable tasks that must be
submitted to a Scheduler without the decoration registered via
Schedulers.onScheduleHook(String, Function). Schedulers.onSchedule(Runnable)
now returns such tasks as-is.

Why

Library-internal, self-rescheduling maintenance tasks (resource eviction and the
like) never execute user code, yet they currently receive the same task decoration
as user tasks. With Hooks.enableAutomaticContextPropagation() enabled, that
decoration captures the scheduling caller's ThreadLocal state into the pending
task; because such tasks re-schedule themselves from inside the restored snapshot
scope, the captured request-scoped state is retained for the lifetime of the
resource (437MB observed in production — see #4338 and reactor/reactor-netty#4299).

There is no way to opt out today: every asynchronous Scheduler built via the
Schedulers factory methods funnels tasks through Schedulers.onSchedule(...).
reactor-netty had to hand-write a bypassing Scheduler in reactor/reactor-netty#4300;
reactor-pool's eviction task has the same shape and still affects direct
reactor-pool users. This marker gives libraries a first-class opt-out while
keeping the standard schedulers, including schedulers supplied by the user.

The name mirrors the existing NonBlocking thread marker in the same package.
Happy to rename or reshape the API if you prefer a different form.

Changes

• New reactor.core.scheduler.UndecoratedRunnable marker interface (no methods).
• Schedulers.onSchedule(Runnable) skips hook application for tasks implementing it.
  The added instanceof sits behind the existing hook != null short-circuit, so it
  only executes when a hook is registered, and is negligible next to the wrapper
  allocation it avoids (the class already type-checks similarly with
  Thread instanceof NonBlocking).
• Tests in SchedulersHooksTest: a direct onSchedule unit test and a
  parameterized end-to-end test across PARALLEL / BOUNDED_ELASTIC / SINGLE /
  EXECUTOR_SERVICE / EXECUTOR verifying the hook is not applied to marked tasks.

User-facing tasks should not implement the marker (documented in the javadoc):
skipping decoration also skips user-registered hooks such as MDC propagation.

Fixes #4338.