feat(plugin): expose server URL to plugins

[EduSantosBrito]

Plugins currently have no way to know the actual server URL. When the server starts on a dynamic port (e.g., if 4096 is unavailable), plugins are unaware of the actual address.

Changes

• Add Server.url() function to retrieve the running server's URL with fallback to http://localhost:4096
• Add serverUrl: URL to PluginInput so plugins receive the actual server address
• Include url in health endpoint response for external clients

Test plan

• [x] bun test passes
• [x] TypeScript compiles without errors