Do not report extremely high HeartbeatLag values until the first heartbeat is read

[grodowski]

Description

Discovered it's possible to log uninitialized time values of lastHeartbeatOnChangelogTime before the first heartbeat arrives.

[gh-ost] : Copy: 0/0 100.0%; Applied: 0; Backlog: 0/1000; Time: 0s(total), 0s(copy); streamer: binlog.1350661:30466581; Lag: 0.00s, HeartbeatLag: 9223372036.85s, State: migrating; ETA: due

In case this PR introduced Go code changes:

• contributed code is using same conventions as original code
• script/cibuild returns with no formatting errors, build errors or unit test errors.

[Copilot]

Pull request overview

This PR prevents HeartbeatLag from showing extremely large values caused by an uninitialized lastHeartbeatOnChangelogTime before the first heartbeat event is processed, by initializing the value when a migration/revert starts.

Changes:

• Initialize lastHeartbeatOnChangelogTime to StartTime at the beginning of Migrator.Migrate() and Migrator.Revert().
• Add assertions in TestRevertEmpty to ensure TimeSinceLastHeartbeatOnChangelog() is reasonable after Migrate() and after Revert().

Reviewed changes

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

File	Description
go/logic/migrator.go	Initializes heartbeat timestamp at start of migrate/revert to avoid reporting uninitialized lag.
go/logic/migrator_test.go	Adds regression assertions to catch uninitialized/huge heartbeat lag durations.

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