test(cli): regression for deferred platform CLI registration path

Responds to hermes-sweeper review on #54717: existing tests only mocked
platform_registry.get(). Add a hermetic fake deferred-loader test that
runs real PlatformRegistry resolution → PluginContext.register_cli_command
→ argparse subparser/choices visibility, without Photon SDK imports.

tests/hermes_cli/test_startup_plugin_gating.py

@@ -249,3 +249,79 @@ def test_deferred_platform_cli_resolution_swallows_registry_errors():
     with patch.dict(sys.modules, {"gateway.platform_registry": fake_module}):
         # Must not raise.
         _resolve_deferred_platform_cli_command("photon")
+
+
+def test_deferred_platform_loader_registers_cli_command_before_parser_table():
+    """Fake deferred loader must resolve through real registry + register CLI
+    before argparse builds plugin command subparsers (issue #54678 review).
+
+    Existing unit tests mock ``platform_registry.get`` and only assert the
+    helper calls it. This regression exercises the full chain hermes-sweeper
+    asked for:
+
+    1. a deferred platform loader is pending on a real ``PlatformRegistry``
+    2. that loader materializes a ``register_cli_command`` side effect on the
+       plugin manager (no Photon SDK import)
+    3. ``_resolve_deferred_platform_cli_command`` runs the pending loader
+    4. the registered command is present on the manager *and* visible as an
+       argparse subparser/choice after the same construction path ``main()``
+       uses for plugin CLI commands
+    """
+    import argparse
+
+    from gateway.platform_registry import PlatformRegistry
+    from hermes_cli.plugins import PluginContext, PluginManager, PluginManifest
+
+    mgr = PluginManager()
+    manifest = PluginManifest(name="fake-photon-platform")
+    command_name = "fakephoton"
+
+    def _fake_loader():
+        # Mirrors what a real platform adapter does on import: register its
+        # top-level hermes <name> CLI command via PluginContext.
+        ctx = PluginContext(manifest, mgr)
+        ctx.register_cli_command(
+            name=command_name,
+            help="Fake deferred platform CLI (test-only)",
+            setup_fn=lambda p: None,
+            description="Hermetic stand-in for deferred platform CLI registration",
+        )
+
+    registry = PlatformRegistry()
+    registry.register_deferred(command_name, _fake_loader)
+
+    # Precondition: deferred is known, but CLI command is not yet registered.
+    assert registry.is_registered(command_name)
+    assert command_name not in mgr._cli_commands
+    assert command_name in registry._deferred
+
+    fake_module = type(sys)("gateway.platform_registry")
+    fake_module.platform_registry = registry
+    with patch.dict(sys.modules, {"gateway.platform_registry": fake_module}):
+        _resolve_deferred_platform_cli_command(command_name)
+
+    # Loader resolution must promote deferred -> concrete and fire CLI register.
+    assert command_name not in registry._deferred
+    assert command_name in mgr._cli_commands
+    cmd_info = mgr._cli_commands[command_name]
+    assert cmd_info["name"] == command_name
+    assert cmd_info["plugin"] == "fake-photon-platform"
+
+    # Same parser-table assembly main() uses after reading _cli_commands.
+    parser = argparse.ArgumentParser(prog="hermes")
+    subparsers = parser.add_subparsers(dest="command")
+    for info in mgr._cli_commands.values():
+        plugin_parser = subparsers.add_parser(
+            info["name"],
+            help=info["help"],
+            description=info.get("description", ""),
+        )
+        info["setup_fn"](plugin_parser)
+        if info.get("handler_fn") is not None:
+            plugin_parser.set_defaults(func=info["handler_fn"])
+
+    # argparse surface: choices + parse success.
+    choices = getattr(subparsers, "choices", None) or {}
+    assert command_name in choices
+    parsed = parser.parse_args([command_name])
+    assert parsed.command == command_name