cos-extensions: upgrade google.golang.org/grpc to v1.83.2

Upgrade google.golang.org/grpc dependency to v1.83.2 and re-vendor Go
dependencies via `go mod vendor` in app-admin/extensions-manager.

Commit 77a5e67759b3 upgraded google.golang.org/grpc from v1.82.2 to
v1.83.1 to resolve CVE-2026-84304, which regressed the fix for
CVE-2026-84445 on the v1.83.x branch. Upgrading to v1.83.2 retains the
CVE-2026-84304 fix and restores the CVE-2026-84445 fix.

Provenance:
- CVE-2026-84445: Go module google.golang.org/grpc fixed in v1.82.2 and
  v1.83.2, fix commits 3822494d8ea0 ("cherry-pick #9365 to v1.82.x
  (#9367)"), 8668b69c167d ("cherry-pick #9365 to v1.83.x (#9366)"),
  93e31b48545e ("transport: reject requests missing :authority and host
  headers (#9365)")
  (https://github.com/advisories/GHSA-2v4p-qf9q-27wj)
- CVE-2026-84304: Go module google.golang.org/grpc fixed in v1.83.1,
  fix commits 7354d9c8debb ("transport: restrict memory overhead of
  buffering small data frames (#9331)"), 8cfeca0e1ee5 ("Cherry-pick
  #9331 to v1.83.x (#9333)")
  (https://github.com/advisories/GHSA-vp52-pcj8-j9qc)

Build adjustments:
- go get google.golang.org/grpc@v1.83.2
- go mod tidy && go mod vendor && go mod verify

BUG=b/581552107, b/579649221
TEST=go mod verify; go test -mod=vendor ./...; go build -mod=vendor
RELEASE_NOTE=None

cos-patch: security-high
Change-Id: I217a6c94759609b233ca91b8cd92d7dc67725272
Reviewed-on: https://cos-review.googlesource.com/c/cos/cos-extensions/+/235848
AI-Review: cloud-image-release <cloud-image-release@prod.google.com>
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>
Reviewed-by: He Gao <hegao@google.com>