Webview message and lifecycle tests

[CatarinaGamboa]

Why: the webview shows what the extension sends it. If the diagnostics or context messages change shape, or Stop/Restart leaves the server in a bad state, nothing catches it today.

What:

• Using onWebviewMessage (Return a small test API from activate() #130): run liquidjava.showView, wait for ready, run Verify, and check the diagnostics and context messages sent to the webview.
• Test liquidjava.stop, liquidjava.start and liquidjava.restart: the status changes, the server process ends on stop, and verification works after restart.

References:

• rust-analyzer's integration tests cover its commands inside a real VS Code: https://github.com/rust-lang/rust-analyzer/tree/master/editors/code/tests/unit