Optimize PropsAdapter + add test coverage

[pjfanning]

Port of akka/akka-core#31695. Reduces heap allocations in PropsAdapter.apply (called on every typed actor spawn) and adds the accompanying test coverage.

part of #2730

PropsAdapter optimizations (PropsAdapter.scala)

• DefaultTypedDeploy constant — pre-built Deploy (local scope + typed default mailbox) reused across calls instead of reconstructed each time
• TypedCreatorFunctionConsumerClazz constant — cached class reference
• Build Deploy directly — replaces chained .withDispatcher() / .withMailbox() / .withDeploy() calls (each producing a new Props copy) with direct .copy() on Deploy
• Fast-path for Props.empty — skips all matching logic and reuses DefaultTypedDeploy directly for the common no-custom-props case
• Direct Props constructor — uses new pekko.actor.Props(deploy, clazz, args) to bypass factory methods and avoid extra intermediate copies

Props caching (CachedProps.scala, Behavior.scala, ActorRefFactoryAdapter.scala)

• CachedProps.scala (new) — internal case class holding a cached typed→classic Props mapping
• Behavior.scala — adds _internalClassicPropsCache volatile field to Behavior[T] so each behavior instance can cache its last adapted classic Props
• ActorRefFactoryAdapter.scala — adds classicPropsFor helper that checks the behavior's cache before adapting props, avoiding repeated work when spawning many actors of the same type with the same props

Additional optimizations

• ActorContextAdapter.scala — lazy _self field: ActorRefAdapter is not allocated unless self is actually accessed
• ActorAdapter.scala — val supervisorStrategy → final override def supervisorStrategy to avoid per-instance field allocation

Test coverage (PropsAdapterSpec.scala)

Added tests covering all PropsAdapter code paths:

• MailboxSelector.defaultMailbox() → "pekko.actor.typed.default-mailbox"
• withDispatcherFromConfig("some.path") → dispatcher "some.path"
• withDispatcherSameAsParent → dispatcher ".."
• MailboxSelector.fromConfig("some.path") → mailbox "some.path"
• MailboxSelector.bounded(24) → mailbox "bounded-capacity:24"
• ActorTags.create("my-tag") → deploy.tags == Set("my-tag")