Return a small test API from activate() #130

Why: integration tests run against the webpack bundle (dist/extension.js), so they can't import state.ts and see the running extension. Polling getDiagnostics can't tell "no errors" apart from "the server crashed" or "it hasn't finished yet". The server catches verifier exceptions, clears diagnostics and sends a failure.

What: return an object from activate(), without changing behaviour:

• ready: a promise that resolves once the language client has started
• onDiagnostics: fires when liquidjava/diagnostics arrives, including when it's empty
• getState(): the current status and diagnostics
• onWebviewMessage: messages sent to and from the webview (used in Webview message and lifecycle tests #133)

References:

• vscode-java exposes serverReady() and onDidServerModeChange on its extension API: https://github.com/redhat-developer/vscode-java/blob/main/src/extension.api.ts