@securelend/sdk@0.1.0-beta.5 does not export webhook helpers.
The default branch of SecureLend/sdk exports verifyWebhook, parseEvent, signWebhook, and signOutcomePayload from packages/sdk/src/webhooks.ts, and WebhookSignatureError, FeatureDisabledError, ConsentRequiredError, and InvalidTransitionError from packages/sdk/src/utils/errors.ts. That code is in package version 0.2.0-beta.0, which is not the published tarball. None of those names are in @securelend/sdk@0.1.0-beta.5.
signOutcomePayload is the unpublished helper whose source comment says it builds the header for POST /v1/webhooks/lender-outcomes: SHA-256 of the secret, a ., and the raw body, returned as sha256= plus the hex digest. The same comment says the live route does not check a timestamp. Signing and verification were not executed for this page, and that route was not called.
The three extra error classes are also unpublished. FeatureDisabledError uses type feature_disabled. ConsentRequiredError uses type consent_required. InvalidTransitionError uses type invalid_transition. They were not thrown against a live server for this page.