Unified sorter manual GC may pause all goroutines and cause gRPC keepalive dog timeout

[overvenus]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do? If possible, provide a recipe for reproducing the error.

Running a changefeed that captures 6k tables.

With Unified Sorter	W/O Unified Sorter
25s	7s

By removed these lines, TiCDC backs to normal.

https://github.com/pingcap/ticdc/blob/284dd55766f4f57b4c3550057c219bd1c92e5331/cdc/puller/sorter/backend_pool.go#L136-L146

2. What did you expect to see?

No keepalive dog timeout.

3. What did you see instead?

keepalive dog timeout

4. Versions of the cluster

release-5.0 and release-5.1

[overvenus]

Following PR ues values in channel types to reduce GC pressure,

• stability-test - 2025-09-30 04:30:15 ticdc#2415
• Data loss when inject randomDropMessage failpoint ticdc#2474

[overvenus]

This issue also relates to pingcap/ticdc#2553 .