ref(webhooks): Show webhooks on integrations directory page

[Christinarlong]

Summary

• When legacy-webhook-ui flag is on, replaces the plugin API's webhook entry in the integrations directory with a synthetic entry backed by GET /organizations/{org}/legacy-webhooks/
• Filters the webhook plugin out of the plugin API response to avoid duplicates
• Renders the webhook row with type="firstParty" and slug="legacy-webhooks" so it links directly to /settings/{org}/integrations/legacy-webhooks/
• Static webhook plugin config lives in legacyWebhookPluginConfig.ts
• Search and category filtering work normally since the entry flows through the same list pipeline

Depends on #117033