Introduce a SourcePrimaryConnectionConfig for primary-required move-table ops

[zacharysierakowski]

re: https://github.com/github/database-infrastructure/issues/8275

Description

This PR does a few things that were lingering on our backlog.

1. Primarily, it adds a separate connection specifically for the source primary. We need this for things like the write cutover RENAME, as well as the post-move-tables cleanup for DROP the source renamed table (if specified by the cli arg)
2. It also condenses the write cutover T1 and T2 ops so that it's safer by running the RENAME and GTID gather in a single DB query using a multi-statement. This is safer because it lessens the chance of crash between the RENAME and the GTID query. And still ensures we're running the two on the same connection.
3. Adds a guard to ensure the operator is providing a pointer to the primary for the TARGET. All target operations require to run against the primary (unlike the source) so this is just a way to provide an early exit
4. Adds another guard to ensure the operator is providing a pointer to a replica (if possible) for the SOURCE. If there are no replicas, or the operator is absolutely sure they want to use the primary, they can pass --allow-on-source-primary to bypass this guard

Demo(s)

Auto resolve source primary from source replica

[Guard] Early exit if target is a replica

[Guard] Early exit if source is a primary and have not specified the new override arg (--allow-on-source-primary)

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.

[zacharysierakowski]

random script i've had stashed for a while that's been handy between local testing so i don't have to do a full teardown/setup.

happy to remove from this PR if we don't want it merged.

[zacharysierakowski]

This was only added recently as part of #1717. I added it knowing that it probably wasn't the right spot for it, but without having the distinction between the primary vs. replica connections (this PR) it was hard to figure out the best spot for this drop operation to dangle from

[Copilot]

Pull request overview

This PR improves --move-tables correctness and safety by introducing a dedicated source-primary connection for primary-required operations (cutover RENAME, drain-GTID capture, and source __del cleanup), while also adding early guards to prevent accidentally running move-tables against non-writable targets or against the source primary for read-heavy phases.

Changes:

• Add a dedicated sourcePrimaryDB/SourcePrimaryConnectionConfig for move-tables primary-required operations, including consolidated RENAME; SELECT @@global.gtid_executed in a single multi-statement round trip.
• Add startup guards to fail fast when the move-tables target is read_only, and to block running the source read path on the source primary unless --allow-on-source-primary is set.
• Update local move-tables scripts/docs to better support primary auto-resolution from a source replica, including /etc/hosts alias assistance and a new reset helper.

Show a summary per file

File	Description
script/move-tables/setup	Adds best-effort /etc/hosts aliasing so host-side gh-ost can resolve docker-compose service names used by replication metadata.
script/move-tables/reset	New helper script to reset source/target tables for local move-tables testing.
script/move-tables/README.md	Updates the example invocation to read from the source replica port and includes --initially-drop-socket-file.
go/logic/streamer.go	Removes source-side cleanup responsibility from the streamer.
go/logic/migrator.go	Introduces source-primary connection/config, consolidated cutover T1/T2 multi-statement, read-path guard, and source __del drop via primary.
go/logic/migrator_move_tables_cutover_test.go	Adds/updates unit+integration coverage for source-primary resolution, writability guard, and source-side drop.
go/logic/migrator_move_tables_cleanup_test.go	Adds coverage for nil source-primary connection error on cleanup.
go/logic/applier.go	Adds a writability guard for move-tables target connections.
go/cmd/gh-ost/main.go	Adds --allow-on-source-primary flag wiring.
go/base/context.go	Extends move-tables context with AllowOnSourcePrimary and SourcePrimaryConnectionConfig.

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• Files reviewed: 10/10 changed files
• Comments generated: 1