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.

shared/management/proto/proxy_service.proto

@@ -237,7 +237,7 @@ message SendStatusUpdateRequest {
   bool certificate_issued = 4;
   optional string error_message = 5;
   // Per-account inbound listener state for the account that owns
-  // service_id. Populated only when --private-inbound is enabled and the
+  // service_id. Populated only when --private is enabled and the
   // embedded client for the account is up. Field numbers >=50 reserved
   // for observability extensions.
   optional ProxyInboundListener inbound_listener = 50;