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

shared/management/proto/proxy_service.proto

@@ -36,12 +36,12 @@ service ProxyService {
   rpc ValidateSession(ValidateSessionRequest) returns (ValidateSessionResponse);
 
   // ValidateTunnelPeer resolves an inbound peer by its WireGuard tunnel IP and
-  // checks the resolved user's access against the service's required groups.
+  // checks the resolved user's access against the service's access_groups.
   // Acts as a fast-path equivalent of OIDC for requests originating on the
   // netbird mesh: when the source IP maps to a known peer in the calling
-  // account and that peer's user is in the service's distribution_groups,
-  // the proxy can issue a session cookie without redirecting through the
-  // OIDC flow. Mirrors ValidateSession's response shape.
+  // account and that peer is in the service's access_groups, the proxy can
+  // issue a session cookie without redirecting through the OIDC flow.
+  // Mirrors ValidateSession's response shape.
   rpc ValidateTunnelPeer(ValidateTunnelPeerRequest) returns (ValidateTunnelPeerResponse);
 }
 
@@ -323,7 +323,7 @@ message ValidateTunnelPeerRequest {
 //   "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
 message ValidateTunnelPeerResponse {
   bool valid = 1;
   string user_id = 2;

shared/management/proto/proxy_service_grpc.pb.go

@@ -36,12 +36,12 @@ type ProxyServiceClient interface {
 	// Called by the proxy after receiving a session token from OIDC callback.
 	ValidateSession(ctx context.Context, in *ValidateSessionRequest, opts ...grpc.CallOption) (*ValidateSessionResponse, error)
 	// ValidateTunnelPeer resolves an inbound peer by its WireGuard tunnel IP and
-	// checks the resolved user's access against the service's required groups.
+	// checks the resolved user's access against the service's access_groups.
 	// Acts as a fast-path equivalent of OIDC for requests originating on the
 	// netbird mesh: when the source IP maps to a known peer in the calling
-	// account and that peer's user is in the service's distribution_groups,
-	// the proxy can issue a session cookie without redirecting through the
-	// OIDC flow. Mirrors ValidateSession's response shape.
+	// account and that peer is in the service's access_groups, the proxy can
+	// issue a session cookie without redirecting through the OIDC flow.
+	// Mirrors ValidateSession's response shape.
 	ValidateTunnelPeer(ctx context.Context, in *ValidateTunnelPeerRequest, opts ...grpc.CallOption) (*ValidateTunnelPeerResponse, error)
 }
 
@@ -201,12 +201,12 @@ type ProxyServiceServer interface {
 	// Called by the proxy after receiving a session token from OIDC callback.
 	ValidateSession(context.Context, *ValidateSessionRequest) (*ValidateSessionResponse, error)
 	// ValidateTunnelPeer resolves an inbound peer by its WireGuard tunnel IP and
-	// checks the resolved user's access against the service's required groups.
+	// checks the resolved user's access against the service's access_groups.
 	// Acts as a fast-path equivalent of OIDC for requests originating on the
 	// netbird mesh: when the source IP maps to a known peer in the calling
-	// account and that peer's user is in the service's distribution_groups,
-	// the proxy can issue a session cookie without redirecting through the
-	// OIDC flow. Mirrors ValidateSession's response shape.
+	// account and that peer is in the service's access_groups, the proxy can
+	// issue a session cookie without redirecting through the OIDC flow.
+	// Mirrors ValidateSession's response shape.
 	ValidateTunnelPeer(context.Context, *ValidateTunnelPeerRequest) (*ValidateTunnelPeerResponse, error)
 	mustEmbedUnimplementedProxyServiceServer()
 }