Fix CommandFailed not emitted when project/document validation fails

[Copilot]

@microsoft/polyglot-notebooks changed its command dispatch so errors thrown inside handlers are now swallowed via .catch() rather than propagated to context.fail(). This broke all guard checks in ProjectKernel — e.g. sending OpenDocument without an open project silently succeeded instead of emitting a CommandFailed event.

Changes

• src/projectKernel.ts — Replace all throw new Error(...) guard patterns with explicit commandInvocation.context.fail(message); return; in every command handler (SubmitCode, OpenDocument, RequestDiagnostics, RequestCompletions, RequestHoverText, RequestSignatureHelp)
• Remove now-dead throwIfProjectIsNotOpened() and throwIffDocumentIsNotOpened() methods

Before:

handle: (commandInvocation) => {
  this.throwIfProjectIsNotOpened(); // thrown error gets swallowed by library
  return this.handleSubmitCode(commandInvocation);
}

After:

handle: (commandInvocation) => {
  if (!this._project) {
    commandInvocation.context.fail(`Project must be opened, send the command '${polyglotNotebooks.OpenProjectType}' first.`);
    return Promise.resolve();
  }
  return this.handleSubmitCode(commandInvocation);
}