fix(review): coderabbit follow-ups

- openapi.yml: declare default: false on ServiceTargetOptions.direct_upstream
  so generated clients/validators reflect the documented default.
- proto/proxy_service.proto: ValidateTunnelPeer doc + denied_reason list
  said "distribution_groups" (bearer-auth field) but the actual gate is
  service.access_groups. Replaced both occurrences to match the code path
  in checkPeerGroupAccess.
- peers/manager.go (GetPeerWithGroups) + users/manager.go (GetUserWithGroups):
  on store error after a successful first lookup, both now return
  (nil, nil, err) so callers can't get a valid entity alongside a non-nil
  error.

Findings skipped with reasons:
- embedded.go merged CLI/Dashboard redirect URIs: pre-existing on
  origin/main, not introduced by this PR.
- account_mock.go MarkPeerDisconnected zero-time UnixNano: same — pre-existing.
- openapi Service schema if/then conditionals: Go-side Validate() already
  enforces these invariants (Private + non-empty AccessGroups, mode=http,
  mutually-exclusive with bearer), and oapi-codegen on OpenAPI 3.1.x
  doesn't honour allOf/if/then anyway.
- *.patch / *.diff / b-n-p.sh: untracked personal artifacts, not part of
  any commit.

shared/management/proto/proxy_service.pb.go

@@ -2185,7 +2185,7 @@ func (x *ValidateTunnelPeerRequest) GetDomain() string {
 //	"no_user"        — peer exists but is not bound to a user
 //	"service_not_found"
 //	"account_mismatch"
-//	"not_in_group"   — user resolved but not in service.distribution_groups
+//	"not_in_group"   — peer resolved but not in service.access_groups
 type ValidateTunnelPeerResponse struct {
 	state         protoimpl.MessageState
 	sizeCache     protoimpl.SizeCache