CAMEL-23281: fix split/aggregate deadlock with SynchronousExecutorService in transacted routes

[Croway]

Summary

• Fix deadlock when using SynchronousExecutorService with split + aggregate in transacted routes
• CAMEL-23030 changed scheduleSync(task) to schedule(task) for SynchronousExecutorService. The aggregated exchange loses the transacted flag (Exchange.copy() doesn't preserve it), so this new branch is always taken. schedule() queues the completion task, but the surrounding transacted split creates nested await → executeFromQueue cycles where Pipeline.scheduleMain() queue swapping makes the task unreachable — deadlock.
• Fix: process the completion task inline via processor.process(exchange, callback) for SynchronousExecutorService, bypassing the reactive queue entirely. No deadlock, no stack overflow.

Test plan

• SplitAggregateInChoiceSynchronousExecutorTest — 3 tests covering non-transacted, transacted, and transacted+choice scenarios
• SplitAggregateSynchronousExecutorStackOverflowIssueTest — CAMEL-23030 regression guard still passes
• SplitAggregateStackOverflowIssueTest and SplitAggregateParallelProcessingStackOverflowIssueTest — existing tests pass

[github-actions]

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[github-actions]

🧪 CI tested the following changed modules:

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