Foundation: Synchronize channel replacement and prevent FileChannel data races

[Tyagiquamar]

Summary

This PR addresses thread-safety issues during dynamic channel replacement and logging in POCO Foundation:

1. Logger Channel Synchronization:

   • Added _channelMutex in Poco::Logger guarding channel assignment (setChannel) and retrieval (getChannel).
   • Logging paths now acquire a local Channel::Ptr reference under getChannel() before invoking pChannel->log(...), ensuring active message logging remains valid even if another thread resets or changes the channel concurrently.

2. FormattingChannel Synchronization:

   • Added mutex protection over _pFormatter and _pChannel accessors and mutators in Poco::FormattingChannel.
   • Thread-safe property forwarding and channel invocation.

3. FileChannel Mutex Scope:

   • Extracted internal unsafeOpen() to ensure file handle initialization and rotation checks occur under the existing _mutex lock in FileChannel::log(), avoiding race conditions between open and log operations across concurrent threads.

4. Unit & Concurrency Tests:

   • Added testConcurrentChannelReplacement and testConcurrentShutdown to Foundation/testsuite/src/LoggerTest.cpp verifying stability under multi-threaded channel mutation.

Validation

• Clean build in Docker (ubuntu:noble).
• Foundation-testrunner LoggerTest executed and passed 100% (9/9 tests OK).