CAMEL-20199: Replace synchronized with ReentrantLock for virtual thread compatibility

[gnodet]

Summary

• Replace synchronized blocks/methods with ReentrantLock or better concurrent data structures across core modules and components to prevent virtual thread pinning (JDK 21+)
• Virtual threads get pinned to carrier threads when entering synchronized blocks, degrading throughput. ReentrantLock does not cause pinning.
• Where possible, use lock-free data structures instead of locks: ConcurrentLinkedQueue, CopyOnWriteArrayList, volatile with double-checked locking

Core changes

File	Change
Suppliers.java	synchronized → ReentrantLock for memoization
DefaultModel.java	9 synchronized methods → single ReentrantLock (reentrant for nested calls)
DefaultCamelContext.java	synchronized(model) → uses DefaultModel's lock

Component changes

Component	Change
camel-aws2-sqs	volatile + ReentrantLock double-checked locking, removed redundant queueUrlInitialized flag
camel-mock	synchronized → ReentrantLock
camel-google-firestore	synchronized + LinkedList → ConcurrentLinkedQueue (lock-free)
camel-google-pubsub	synchronizedList → CopyOnWriteArrayList (lock-free)
camel-milo	12 synchronized constructs → ReentrantLock across SubscriptionManager, MiloClientConnection, CamelNamespace

Intentionally skipped

• CxfConsumer: synchronizes on per-request continuation object — brief pinning, CXF API constraint
• KameletComponent, CamelWebSocketHandler: already use ReentrantLock
• XML converters: per-document synchronization, brief lock scope
• ThreadLocal usages: same-thread semantics, work correctly with virtual threads

Test plan

• Existing unit tests pass for all modified modules
• CI build passes (formatting, sourcecheck)
• Verify no regressions in core routing tests

🤖 Generated with Claude Code

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