Use async cache lookup for suspending functions

[sugowslt]

Closes gh-36644

Summary

• Route Kotlin suspending @Cacheable functions through the asynchronous Cache.retrieve() lookup path.
• Keep the existing reactive adapter behavior and synchronous lookup behavior for ordinary methods.
• Add regression coverage for cache misses and hits on suspending and ordinary functions.

Testing

• ./gradlew :spring-context:test --tests org.springframework.cache.KotlinCacheAsyncLookupTests --no-daemon --no-scan --max-workers=2
• ./gradlew :spring-context:check --no-daemon --no-scan --max-workers=2