Cleanup and document throttlers

[morgo]

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This is a small code cleanup:

• Rename test throttler to mock throttler.
• Simplify code now that we have long supported only mysql 8.0.
• Add documentation on throttlers.

[Copilot]

Pull request overview

This PR cleans up and documents the throttler package by renaming the test throttler to "Mock", simplifying code to support only MySQL 8.0+, and adding comprehensive documentation.

Changes:

• Renamed Test throttler to Mock throttler for clearer semantics
• Consolidated MySQL80Replica into Replica and removed the Repl base struct, simplifying the code now that only MySQL 8.0+ is supported
• Added comprehensive README documentation for the throttler package with usage examples and extension guidelines

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
pkg/throttler/mock.go	Renamed Test struct to Mock and updated all method receivers
pkg/throttler/throttler_test.go	Added comprehensive tests for the Mock throttler including context cancellation
pkg/throttler/throttler.go	Updated NewReplicationThrottler to directly return Replica instead of MySQL80Replica, simplified documentation
pkg/throttler/replica.go	Consolidated code from deleted repl.go, renamed MySQL80Replica to Replica, and moved fields directly into the struct
pkg/throttler/repl.go	File deleted - functionality merged into replica.go
pkg/throttler/README.md	New comprehensive documentation covering design philosophy, implementations, usage, and extension guidelines
pkg/migration/runner.go	Updated reference from throttler.Test{} to throttler.Mock{}

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