downstreamadapter: skip obsolete replayed events

[asddongmen]

What problem does this PR solve?

Issue Number: close #5421

Dispatcher reset, dynstream path release, and DDL/syncpoint replay can reuse stale commit-ts state or reprocess completed block events.

What is changed and how it works?

This PR resets event-collector commit-ts deduplication when a dispatcher enters a new epoch, resets the current event-service session after dynstream releases queued events, and tracks completed DDL/syncpoint block events so obsolete DML/block events are skipped.

The EventService deleted-table scan fix has been split into #5489.

Check List

Tests

• Unit test

Questions

Will it cause performance regression or break compatibility?

No. The changes only affect replay and stale-event handling in reset/completed-block paths.

Do you need to update user documentation, design documentation or monitoring documentation?

No.

Release note

Fix dispatcher replay handling to skip obsolete events after reset or completed DDL/syncpoint progress.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

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Run ID: 0eb90d03-1c7a-4471-af33-9d0ee14ee341

📥 Commits

Reviewing files that changed from the base of the PR and between be31b9a and 024a92e.

📒 Files selected for processing (7)

• downstreamadapter/dispatcher/basic_dispatcher.go
• downstreamadapter/dispatcher/basic_dispatcher_active_active_test.go
• downstreamadapter/dispatcher/helper.go
• downstreamadapter/dispatcher/helper_test.go
• downstreamadapter/eventcollector/dispatcher_stat.go
• downstreamadapter/eventcollector/dispatcher_stat_test.go
• downstreamadapter/eventcollector/event_collector.go

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[gemini-code-assist]

Code Review

This pull request introduces mechanisms to skip obsolete DML and block events that are already completed, resets event deduplication filters when advancing epochs for reset, and resets the event service upon releasing dispatcher memory. Additionally, it fixes a potential no-progress loop in the event scanner when scanning deleted tables by finalizing the scan at the raw event's commit timestamp instead of subtracting one. There are no review comments to address, and I have no additional feedback to provide.

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[asddongmen]

/test all

[asddongmen]

/test all

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[lidezhu]

Can you add a concrete example to explain why these three lines is necessary?