Use `tool` directive in `go.mod` to specify mockgen after upgrading to Go 1.24

[qdm12]

Context and scope

#3628 adds a tools.go with a blank import at the project root to satisfy dependency requirements for mockgen@v0.5.
Once Go 1.25 is released, we can upgrade to 1.24 and use the tool directive in go.mod (see this).

[github-actions]

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