Fix RotateEvent and flaky tests

[morgo]

A Pull Request should be associated with an Issue.

We wish to have discussions in Issues. A single issue may be targeted by multiple PRs.
If you're offering a new feature or fixing anything, we'd like to know beforehand in Issues,
and potentially we'll be able to point development in a particular direction.
Further notes in https://github.com/block/spirit/blob/main/.github/CONTRIBUTING.md

The first issue:
The cause is the cancel() running too early and leaving a context cancelled error. If we run it after Close() is ensures a cleaner finish. Fixes #552

The second issue:
Spirit incorrectly handles RotateEvent, leading to events potentially being skipped in resuming from a checkpoint. Fixes #548

This can be a serious issue, but the steps required to reproduce it are quite specific. See #548 for discussion on it. I will merge this, deploy it to our staging environment and then likely cherry pick a release-branch version of it.

[Copilot]

Pull request overview

This PR fixes two flaky test issues: (1) race conditions in tests caused by calling cancel() before Close(), and (2) timeout issues during the cutover phase due to an excessively high trivial threshold for unflushed changes.

• Reduced binlogTrivialThreshold from 10,000 to 1,000 changes to prevent flush timeouts during the cutover lock phase
• Reordered Close() to execute before cancel() in all test functions to avoid context cancellation race conditions
• Added comprehensive documentation explaining the tradeoffs of the lower threshold value

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
pkg/repl/client.go	Reduced binlogTrivialThreshold constant from 10,000 to 1,000 and added detailed documentation explaining the tradeoff between cutover responsiveness and lock duration
pkg/migration/runner_resume_test.go	Fixed test flakiness by calling Close() before cancel() in four test functions (TestCheckpointResumeDuringChecksum, TestResumeFromCheckpointE2E, TestResumeFromCheckpointCompositeVarcharPK, TestResumeFromCheckpointStrict, TestResumeFromCheckpointE2EWithManualSentinel) to prevent race conditions

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

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