Test webview messages and server lifecycle

[rcosta358]

Adds real VS Code coverage for webview readiness, diagnostics/context messages, and Stop, Start, and Restart. Checks process termination and verification after Restart, and fixes a shutdown race that could clear the newly started server.

Validated both fixtures locally and in CI on stable and VS Code 1.82.0; lint, types, and installation passed.

Depends on #145. Closes #133.

Generated by Codex.

[CatarinaGamboa]

Two assertions here can't catch the regression they're meant to catch.

Reviewed with Claude Code (reviewer + adversarial agents per PR, findings checked against the code before posting).

[CatarinaGamboa]

This can match the old context. handleLJDiagnostics sends a context message with the cached extension.context from the previous verification, so this listener can resolve on that message, not on the context Verify produces. The fixture doesn't change between runs, so the old and new contexts look the same, and the assertions on valid pass even if Verify stopped sending a context.

Suggest waiting for the context message that follows the liquidjava/context notification for this run, for example by clearing the cached context before Verify, or by matching on something that changes per run.

[CatarinaGamboa]

This compares an array with itself. handleLJDiagnostics passes the same diagnostics array to sendMessage and to diagnosticsEmitter.fire, and sendMessage fires onWebviewMessage with that object before postMessage. So outboundDiagnostics.message.diagnostics and diagnostics are the same reference, and deepEqual always passes. To check what the webview receives, compare against a copy taken before sending, or check specific fields such as the error type and file.