fix: align sidecar builder go version

[acmore]

Summary

• update infra/sidecar/Dockerfile to use Go 1.26 in the builder stage so it matches go.mod
• add a regression test that fails if the sidecar builder Go version drifts from the go.mod Go version again
• fix the main-branch e2e failure that stopped at sidecar image build before any e2e steps ran

Verification

• GOCACHE=/tmp/okdev-go-cache-sidecarfix go test ./infra/sidecar -run TestBuilderGoVersionMatchesGoMod -count=1
• HOME=/tmp/okdev-home-sidecarfix GOCACHE=/tmp/okdev-go-cache-sidecarfix GOMODCACHE=/tmp/okdev-go-mod-sidecarfix GOPATH=/tmp/okdev-go-path-sidecarfix go vet ./...
• HOME=/tmp/okdev-home-sidecarfix GOCACHE=/tmp/okdev-go-cache-sidecarfix GOMODCACHE=/tmp/okdev-go-mod-sidecarfix GOPATH=/tmp/okdev-go-path-sidecarfix go test ./...
• /Users/jacky.he/work/okdev/.venv/bin/pre-commit run --all-files --hook-stage manual okdev-gofmt

Notes

• local docker build could not be re-run here because no Docker daemon is available at /Users/jacky.he/.colima/default/docker.sock; the failing GitHub Actions step was used as the root-cause repro instead