Fixes the cutover rename+gtid selection and drain logic

[zacharysierakowski]

re: https://github.com/github/database-infrastructure/issues/8209
targets: #1704

Description

Applies some suggestions and fixes on top of #1704.

Details

• fixes the rename+gtid selection so that it's on the same connection as mentioned in the spec doc
• fixes drain logic to also check the queue so it doesn't exit the cutover too early
• reuses CutOverLockTimeoutSeconds instead of using a new, hard-coded constant (defaults to 60 seconds still)
• makes --postpone-cut-over-flag-file required for --move-tables
• adds helper script for inserting rows to the source continuously

[Copilot]

Pull request overview

This PR tightens correctness and operator-safety of move-tables cooperative cutover by ensuring rename+GTID capture are connection-pinned, drain completion also accounts for queued work, and move-tables runs are gated by an explicit postpone flag file (with a move-tables-appropriate cutover timeout default).

Changes:

• Pin T1 (RENAME) + T2 (GTID capture) to a single DB connection and improve T3 drain completion logic by also checking streamer/apply backlogs.
• Make --postpone-cut-over-flag-file required for --move-tables, and default --cut-over-lock-timeout-seconds to 60s in move-tables mode (while preserving explicit user overrides).
• Update local move-tables scripts/docs and add a helper script to continuously insert rows on the source primary.

Show a summary per file

File	Description
script/move-tables/setup	Forces local replicas to be read-only to prevent accidental replica-only mutations and to fail cutover rename when pointed at a replica.
script/move-tables/README.md	Updates example invocation for move-tables to use primary and require postpone flag file.
script/move-tables/insert-source-primary-loop	Adds a helper script to generate continuous source-side writes for testing drain/cutover behavior.
go/logic/migrator.go	Updates move-tables orchestration: earlier flag-file creation, T1/T2 pinning, drain timeout reuse, and backlog-aware drain completion.
go/logic/migrator_move_tables_cutover_test.go	Adjusts drain-timeout test to use CutOverLockTimeoutSeconds and adds coverage for “queued DML prevents premature drain success”.
go/cmd/gh-ost/main.go	Enforces required postpone flag for move-tables and sets move-tables default cutover-lock-timeout to 60s unless user specified.
go/base/context.go	Allows CutOverLockTimeoutSeconds up to 60s in move-tables mode.
go/base/context_test.go	Adds tests asserting timeout range differs by mode (standard vs move-tables).

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• Files reviewed: 8/8 changed files
• Comments generated: 1