Z2M: mark custom action extend as isModernExtend

z-h-c's prepareDefinition rejects any item in a definition's `extend` array
that lacks `isModernExtend: true` ("has legacy extend in modern extend"), so
the raw {fromZigbee, toZigbee, exposes} block for the action converter was
rejected and the whole file ignored — even after the e.presets.action fix.

Add isModernExtend: true so z-h-c treats it as a modern extend and merges its
fromZigbee/exposes. Verified by running every generated definition (147 full,
6 trimmed) through zigbee-herdsman-converters 26.85.0 prepareDefinition — the
function Z2M's addExternalDefinition calls — with zero failures.

This and the e.presets.action fix were both latent in the multistate proposal's
converter, which was never regenerated or loaded, so neither was ever exercised.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: neneonline

helper_scripts/templates/switch_custom.js.jinja

@@ -520,7 +520,11 @@ const definitions = [
                     {%- set _ = action_values.append(switchName ~ '_' ~ s) -%}
                 {%- endfor -%}
             {%- endfor -%}
+            // Custom modern-extend: emits the friendly `action` from the raw
+            // multistate value. isModernExtend marks it so z-h-c merges it rather
+            // than rejecting the definition as a legacy extend.
             {
+                isModernExtend: true,
                 fromZigbee: [{
                     cluster: 'genMultistateInput',
                     type: ['attributeReport', 'readResponse'],