cos-extensions: upgrade grpc to v1.79.3 and vendor Go dependencies

1. Upgrade google.golang.org/grpc dependency to v1.79.3 to resolve
   CVE-2026-33186 (cherry-pick of cos/194246).
2. Vendor Go module dependencies via `go mod vendor` so app-admin/extensions-manager
   can build offline (-mod=vendor) as a cros-workon 9999 live ebuild without a
   separate -deps.tar.xz distfiles archive.

BUG=b/541083274, b/561624065
TEST=go test -mod=vendor ./..., CGO_ENABLED=0 go build -mod=vendor
RELEASE_NOTE=Fixed CVE-2026-33186 in cos-extensions.

cos-patch: security-moderate
Change-Id: Ie24f75acfd21f3be2f76a2bd1d6cbe96bad77b53
Reviewed-on: https://cos-review.googlesource.com/c/cos/cos-extensions/+/208067
SLSA-Policy-Verified: SLSA Policy Verification Service <devtools-gerritcodereview-exitgate@google.com>
Tested-by: Miri Amarilio <mirilio@google.com>
AI-Review: cloud-image-release <cloud-image-release@prod.google.com>
Reviewed-by: Robert Kolchmeyer <rkolchmeyer@google.com>