Restore original HTTPRoute spec on Gateway API canary finalize

[pujitha24]

Motivation:
When a Canary using the Gateway API provider (gatewayapi:v1) is deleted
with spec.revertOnDeletion: true, Flagger reverts the target Deployment
and the Kubernetes Service, but GatewayAPIRouter.Finalize was a no-op
that returned nil without touching the HTTPRoute. The HTTPRoute was
left pointing at the Flagger-managed -primary/-canary backends and
carrying the session-affinity cookie rule, both of which reference
Services that are deleted during finalization. This leaves the route
with unresolved backends, causing request failures until an operator
manually restores it.

Approach:
Apply the same annotation-based revert pattern already used by
IstioRouter.Finalize (VirtualService) and KubernetesDefaultRouter.Finalize
(Service) to GatewayAPIRouter:

• In Reconcile, the first time Flagger overwrites a pre-existing
  HTTPRoute's spec, the original spec is preserved: either the
  kubectl.kubernetes.io/last-applied-configuration annotation already
  carries it (when the route was applied with kubectl), or Flagger
  stores it under its own flagger.kubernetes.io/original-configuration
  annotation before the first update.
• Finalize now looks up either annotation and, if found, restores the
  HTTPRoute's spec from it. If neither annotation is present (e.g. the
  HTTPRoute was created by Flagger itself and there is nothing to
  revert to), it logs a warning and leaves the route untouched, as
  before.

This change is scoped to pkg/router/gateway_api.go (the gatewayapi:v1
provider named in the report). GatewayAPIV1Beta1Router
(gateway_api_v1beta1.go) is a separate, independently implemented type
with its own copy of Reconcile/Finalize and has the same gap, but is
left untouched here to keep this change minimal and surgical.

Validation:

• go build ./pkg/router/...
• go test ./pkg/router/... (all tests pass)
• go vet ./pkg/router/... (clean)
• gofmt -l on both changed files (clean)
• Added TestGatewayAPIRouter_Finalize with four subtests: HTTPRoute not
  found returns an error; no stored annotation is a graceful no-op;
  restoring from the flagger config annotation; restoring from the
  kubectl annotation. Verified these are a genuine regression test by
  stashing the gateway_api.go change and re-running: 3 of 4 subtests
  fail against the previous no-op Finalize, and all 4 pass with the
  fix applied.
• Interface compliance (router.Interface.Finalize) is verified by
  go build ./pkg/router/..., since pkg/router/factory.go assigns
  &GatewayAPIRouter{} to that interface within the same package.
• Could not run a full-repo go build ./... or the make-based CI
  target in this sandbox due to available disk space being exhausted
  by unrelated heavy dependencies (client-go informers, cloud SDKs)
  pulled in by other packages; the change itself is fully contained
  to pkg/router and was validated at that scope.

Report: #1974
Signed-off-by: Pujitha Paladugu 10557236+pujitha24@users.noreply.github.com
Assisted-by: claude-sonnet-5 (via Claude Code)

Fixes #1974