chore(api,ci,docs,test): private-service schema, proto-check, fixups

Non-functional cleanups and contract/CI hardening around the
private-service work:

API schema (openapi.yml):
- Require a non-empty access_groups and mode=http when private=true,
  on both Service and ServiceRequest, mirroring
  validatePrivateRequirements. mode stays optional-but-constrained
  (empty defaults to http server-side), matching runtime.

CI (proto-version-check.yml):
- Cover renamed .pb.go files (read base via previous_filename).
- Match protoc-gen-go-grpc version headers (optional "- " prefix and
  -gen-go-grpc suffix) so grpc-generated files are in scope.

Docs / comments:
- Reword Config field docs to say defaults are applied at Server.Start
  (initDefaults), not New.
- Rename the obsolete --private-inbound flag to --private across
  comments and the proto doc.

Pre-existing test fixups surfaced by review:
- Repair the integration-tagged validate_session_test.go (SignToken
  signature growth + new Manager interface methods).
- Fix the CI-skip boolean precedence so Windows isn't skipped
  unconditionally.
- Guard the router.HTTPListener type assertion with comma-ok.

proxy/internal/debug/handler.go

@@ -131,7 +131,7 @@ func (h *Handler) SetCertStatus(cs certStatus) {
 
 // SetInboundProvider wires per-account inbound listener observability.
 // Pass nil (or skip the call) to keep the inbound section out of debug
-// responses on proxies that don't run --private-inbound.
+// responses on proxies that don't run --private.
 func (h *Handler) SetInboundProvider(p InboundProvider) {
 	h.inbound = p
 }

proxy/internal/tcp/router_test.go

@@ -1781,11 +1781,14 @@ func TestRouter_PlainHTTP_RoutesToPlainChannel(t *testing.T) {
 		}
 	}()
 
+	tlsListener, ok := router.HTTPListener().(*chanListener)
+	require.True(t, ok, "router.HTTPListener() must be the test's chanListener; the test relies on observing its channel directly")
+
 	select {
 	case conn := <-acceptDone:
 		require.NotNil(t, conn)
 		_ = conn.Close()
-	case <-router.HTTPListener().(*chanListener).ch:
+	case <-tlsListener.ch:
 		t.Fatal("plain HTTP request leaked into TLS channel")
 	case <-time.After(3 * time.Second):
 		t.Fatal("plain HTTP connection never reached plain channel")