Make SingletonScope scoped Provider a static nested class

[arimu1]

Summary

SingletonScope.scope() previously returned an anonymous (non-static) Provider. In Java, that always retains an implicit outer reference to SingletonScope.this, even though the provider only needs key and creator (plus static scope state).

If such a provider is retained from a long-lived structure (e.g. thread pool / ThreadLocal context), that outer reference can keep the scope instance reachable longer than necessary.

This change:

• Replaces the anonymous provider with a private static nested SingletonProvider
• Passes key / creator explicitly through the constructor
• Preserves singleton semantics (double-checked locking, cycle locks, circular proxies, null sentinel, toString)

Fixes #1929

Test plan

• mvn -pl core test -Dtest=ScopesTest,CircularDependencyTest,EagerSingletonTest (JDK 21)
  • ScopesTest 35/35
  • CircularDependencyTest 24/24
  • EagerSingletonTest 4/4
• New testSingletonScopedProviderDoesNotRetainOuterScope asserts static nested class and no this$ synthetic field, plus still caches instances

Notes

• Google CLA may be required for this contribution (human sign-off if the CLA bot flags it).