Fix Migrate() hang on panic abort during throttling

[jakubpliszka]

Description

This PR makes consumeRowCopyComplete() context aware so it always unblocks and exits cleanly on abort.

When gh-ost is panic aborted while throttled, the migration never exits because consumeRowCopyComplete() blocks forever waiting on rowCopyComplete. This happens when context cancellation causes the completion signal to be dropped, leaving teardown and shutdown logic unreachable.

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 fixes a hang in Migrator.consumeRowCopyComplete() that occurred when gh-ost was panic-aborted during throttling. Previously, the function blocked unconditionally on rowCopyComplete, so if the abort path drained/cancelled the migration before any value was sent on that channel, teardown could never proceed. The fix wraps the receive in a select that also returns when the migration context is cancelled.

Changes:

• Make consumeRowCopyComplete() context-aware via select on rowCopyComplete and migrationContext.GetContext().Done().
• Preserve existing behavior of calling mgtr.abort(err) and returning without marking row copy complete on error.

Show a summary per file

File	Description
go/logic/migrator.go	Adds a context-cancellation case to the initial receive in consumeRowCopyComplete() so it unblocks on abort.

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