build(backend): bump Go to 1.26.9 and mockery to 3.8.1

[DoDiODev]

Summary

Patch-level toolchain update for the backend: Go 1.26.6 → 1.26.9 and mockery 3.7.4 → 3.8.1, at every place that pins them.

Changes

Tool	Where	Change
Go	backend/scripts/install-go.sh, devops/docker/lake-builder/Dockerfile	1.26.6 → 1.26.9, new SHA256 for linux-amd64 / linux-arm64 (from go.dev)
Go	workflow step names in test.yml, test-e2e.yml (2×), golangci-lint.yml, migration-script-lint.yml	label only
mockery	backend/scripts/install-mockery.sh	3.7.4 → 3.8.1, new SHA256 for Linux x86_64 / arm64 (from the release's checksum.txt)
mockery	backend/Makefile (MOCKERY_VERSION), backend/Dockerfile, backend/Dockerfile.local, devops/docker/lake-builder/Dockerfile	v3.7.4 → v3.8.1
mockery	workflow step names in test.yml, golangci-lint.yml	label only

Not changed: the go 1.26.0 directive in go.mod, and the golang:1.26-bookworm / devcontainers/go:1-1.26-bookworm base images, which already track the latest 1.26 patch. Go 1.27 is left for a separate change.

golangci-lint stays at v2.12.2. v2.14.0 ships a staticcheck that reports SA1019 without quoting the import path, so the kiro exclusion added in #9197 no longer matches, and it newly flags three uses of the deprecated RawDataSubTaskArgs.Params field. Both deserve their own change.

Generated mocks

With mockery 3.8.1, make mock produces the same 67 mocks. The only difference to 3.7.4 is that the generated constructors and their cleanup now call t.Helper() when available, so assertion failures point at the test instead of the mock file.

Verification

Check	Result
SHA256 of the Go and mockery archives, downloaded and compared with the values in the scripts	✅
make mock, make build, make unit-test with Go 1.26.9	✅
go mod tidy leaves go.mod / go.sum unchanged	✅
golangci-lint v2.12.2	✅ 0 issues
Full CI mirror on my fork, including a build of the lake-builder image from this Dockerfile, unit tests, lint and e2e on MySQL and PostgreSQL (run)	✅

The mericodev/lake-builder change only takes effect once a new builder-* image is published; CI installs Go and mockery through the scripts above and therefore already tests the new versions.

[DoDiODev]

A correction to the description: it says golangci-lint v2.14.0 "newly flags three uses" of the deprecated RawDataSubTaskArgs.Params. Those three were only what the default output showed (max-same-issues: 3). Without that limit, v2.14.0 reports 201 uses across about 40 plugins, all the same SA1019 message. v2.12.2 reports none.

This doesn't change anything in this PR. golangci-lint stays at v2.12.2 here, and the bump will come as its own PR once this one is through.

[klesh]

LGTM.
Thanks for your contribution.