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.

management/internals/modules/peers/manager.go

@@ -113,7 +113,9 @@ func (m *managerImpl) GetPeerByTunnelIP(ctx context.Context, accountID string, i
 	return m.store.GetPeerByIP(ctx, store.LockingStrengthNone, accountID, ip)
 }
 
-// GetPeerWithGroups returns the peer plus its group memberships.
+// GetPeerWithGroups returns the peer plus its group memberships. Any store
+// error returns (nil, nil, err) so callers never receive a valid peer
+// alongside a non-nil error.
 func (m *managerImpl) GetPeerWithGroups(ctx context.Context, accountID, peerID string) (*peer.Peer, []*types.Group, error) {
 	p, err := m.store.GetPeerByID(ctx, store.LockingStrengthNone, accountID, peerID)
 	if err != nil {
@@ -121,7 +123,7 @@ func (m *managerImpl) GetPeerWithGroups(ctx context.Context, accountID, peerID s
 	}
 	groups, err := m.store.GetPeerGroups(ctx, store.LockingStrengthNone, accountID, peerID)
 	if err != nil {
-		return p, nil, err
+		return nil, nil, err
 	}
 	return p, groups, nil
 }