GROOVY-12280: hold cached awaitable adapters through SoftReferences

[paulk-asert]

A ClassValue association lives as long as its key class, and the common keys here are platform classes such as CompletableFuture, so the cached Groovy-loaded adapter pinned Groovy's class loader for the lifetime of the JVM (JDK-8136353 behavior, working as intended). The value is now held through a SoftReference: the association strongly reaches only java.base objects. A cleared reference is removed and recomputed once; should the fresh reference already be cleared, the answer comes from an uncached scan, so lookups terminate under any memory pressure. A NO_ADAPTER sentinel keeps unsupported types distinguishable from cleared references.

The site deliberately stays on java.lang.ClassValue rather than routing through GroovyClassValueFactory: with soft values nothing strongly Groovy-loaded remains in the association, so there is nothing left for the groovy.use.classvalue escape hatch to release here.

Soft references are cleared by the collector under heap pressure, so the class loader becomes collectible rather than instantly collected — the designed semantics of the strategy agreed in the GROOVY-12142 review discussion, not a gap.

The change was also verified against the groovy-concurrent-java deliverable, whose jar is a filtered extraction of core promising Java users no Groovy-runtime dependency: jdeps over the registry classes with only that jar on the classpath resolves every reference from java.base or the jar itself (SoftReference and ClassValue are java.base). That check is a second, independent reason the factory-routing alternative was declined — GroovyClassValueFactory lives in org.codehaus.groovy.reflection, outside the extraction set, so routing through it would have shipped the extracted jar with an unresolvable reference.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 78.94737% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.1852%. Comparing base (1c2f0f2) to head (359464e).
⚠️ Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
...va/groovy/concurrent/AwaitableAdapterRegistry.java	78.9474%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@                Coverage Diff                 @@
##               master      #2818        +/-   ##
==================================================
+ Coverage     70.1794%   70.1852%   +0.0058%     
- Complexity      35853      35882        +29     
==================================================
  Files            1563       1563                
  Lines          132546     132575        +29     
  Branches        24385      24397        +12     
==================================================
+ Hits            93020      93048        +28     
  Misses          31113      31113                
- Partials         8413       8414         +1     

Files with missing lines	Coverage Δ
...va/groovy/concurrent/AwaitableAdapterRegistry.java	72.0000% <78.9474%> (-1.0159%)	⬇️

... and 8 files with indirect coverage changes

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