cdc: add solution for kafka broken pipe error

[wlwilliamx]

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What is changed, added or deleted? (Required)

Added a new FAQ for TiCDC Kafka synchronization scenarios. It explains that a short Kafka idle connection timeout may cause changefeed failures, and suggests adjusting the configuration to avoid the issue. It also clarifies that the problem does not affect continued operation, and only causes a brief lag spike during changefeed restarts.

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• [ ] v6.1 (TiDB 6.1 versions)
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[wlwilliamx]

/retest

[wlwilliamx]

/retest

[ti-chi-bot[bot]]

[LGTM Timeline notifier]

Timeline:

• 2025-08-22 08:18:19.730271584 +0000 UTC m=+600707.673447100: :ballot_box_with_check: agreed by qiancai.

[qiancai]

/approve

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #20824.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.5: #20825.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #20826.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #20827.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #20828.