issue: #46451
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## Summary by CodeRabbit
* **New Features**
* Session versioning added to validate coordinator compatibility during
registration and active takeover.
* **Changes**
* Active–standby flow simplified: standby-to-active activation now
always enabled and initialized unconditionally.
* Registration uses version-aware transactions to ensure version
consistency during takeover.
* Startup/health startup path streamlined.
* **Tests**
* Added version-key integration test; removed test for disabling
active-standby.
* Updated flush test to assert rate-limiter errors occur.
* **Chores**
* Removed centralized connection manager and its test suite.
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Signed-off-by: chyezh <chyezh@outlook.com>