fix: respond 500 instead of crashing when login initiation fails

[cuzzlor]

Stack 2/3 — stacked on #17 (base branch chore/npm-audit); the diff shows only this PR's commit. Merge #17 first, then this, then #16. The version bump for the release ships at the top of the stack in #16.

Problem

createLoginHandler's promise chain ends in a terminal .catch((error) => { throw error }). Rethrowing inside a terminal catch converts the error into an unhandled promise rejection, which terminates the Node process (default behaviour since Node 15).

Any failure while initiating an interactive login — e.g. a transient Entra metadata fetch error inside msalClient.getAuthCodeUrl, or a throwing authorizationUrlRequestOverride — is reachable by an anonymous GET with accept: text/html, so a single request during an Entra blip takes the whole server down.

Fix

Log the error via the configured logger and respond 500, matching createAuthHandler's existing error handling. logger is now threaded through AuthInput to createLoginHandler for this purpose (no public API change — AuthConfig already has logger).

Testing

Added src/login-handler.spec.ts (first tests in the repo — vitest was already wired up):

• login initiation failure responds 500 (under the old code this test run dies on the unhandled rejection)
• successful initiation still redirects to the auth code URL

npm run lint, npm run check-types, and npm test all pass.

🤖 Generated with Claude Code

[cuzzlor]

CI here fails only on the audit step — advisories pre-dating this branch (main fails identically). #17 clears them; update this branch after it merges and CI should go green.