Yahoo's guided app-password setup pointed appPasswordHelpUrl at the
generic account-security sign-in page, which assumes the user already
knows to hunt for "Create app password" once there. Point it instead at
Yahoo's own step-by-step "Generate and manage 3rd-party app passwords"
article, mirroring what #153 did for iCloud.
Yahoo does NOT gate app-password creation behind two-step verification
(verified against Yahoo's live help docs, which never list it as a
prerequisite), so — unlike Gmail and iCloud — it keeps twoFactorHelpUrl
null and shows no 2FA button. AppPasswordSetupScreen already renders the
help buttons generically from these provider fields, so no screen change
is needed; the existing PR #152 ordering (2FA link before the
app-password link) is preserved for the providers that have both.
Add a MailProviderTest assertion pinning Yahoo's new app-password URL
(mirroring the iCloud test) and extend the onboarding
yahooSetup_hasNoTwoFactorHelpLink coverage note for #155.
Closes#155
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>