Fix ConcurrentHashMap contention in ExtensionHolderFactoryImpl

[rdara]

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Overview

Replace computeIfAbsent() with putIfAbsent pattern to avoid bin-level locking during value creation. This prevents thread contention issues where threads get stuck in ConcurrentHashMap.helpTransfer() under high concurrency scenarios.

The fix uses a double-check pattern:

1. Fast path check with get()
2. Create value outside any lock
3. Atomic putIfAbsent - only one thread wins

Trade-off: Under race conditions, multiple threads may create extension instances, but only one wins. This is safe because valueLoader creates empty/stateless instances.

This issue was observed in production environments with high thread counts causing application hangs due to ConcurrentHashMap internal contention.

Ref: NewRelic Support Case #00311074

Related Github Issue

N/A - Issue reported via NewRelic Support Case #00311074

Testing

 - Verified fix resolves thread contention in production environment with high thread counts
 - Existing test suite should pass as the change maintains the same functional behavior
 - 

Checks

• Your contributions are backwards compatible with relevant frameworks and APIs.
• Your code does not contain any breaking changes. Otherwise please describe.
• Your code does not introduce any new dependencies. Otherwise please describe.

[CLAassistant]

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[jtduffy]

@rdara
Thanks for the contribution! This will be in our next release (v9.2.0) which should be available by the end of this week or early next week, assuming no surprises.