Load @azure/msal-node lazily in SharePointTransport

[llbbl]

Deferred from #5 (see PR #12).

src/sharepoint.ts imports ConfidentialClientApplication from @azure/msal-node at the top level, so the SDK loads for every consumer of the package — including ones that only convert Markdown to DOCX and never touch SharePoint.

GoogleDriveTransport now loads @googleapis/drive lazily via a memoized await import() inside upload(). This restores parity.

Measured before the Google transport went lazy: importing the barrel cost ~186ms of module init, of which ~86ms was the Drive SDK alone.

Tasks

• Move the @azure/msal-node import into a memoized lazy loader, mirroring importDriveFilesClient in src/google.ts (including the deliberate no-memoize-on-failure reset).
• Preserve the current constructor-time config validation — only the SDK load moves, not the option checks.
• Add coverage for the memoization and the failed-import retry, mirroring the two tests added for the Drive client.

Partly obviated if #13 (subpath exports) lands first — worth sequencing the two.