Add HTTP basic-auth credential handling for the built-in browser:
auto-fill from the browser password manager, an optional "Remember
this password" prompt, and a password-manager picker to answer a
challenge. Credentials are stored as ordinary browser password-manager
entries (keychain/1Password/etc.), viewable and removable from the new
Saved Passwords section in browser settings.

Acceptance is read only from the user's own main-frame response, so a
remembered credential is persisted only after a 2xx and a known-bad one
is never silently re-supplied. The correlation logic lives in a pure,
WebKit-free state machine (BrowserBasicAuthCoordinator) that is unit
tested without WebKit; iTermBrowserBasicAuthManager is the WebKit/AppKit
adapter that owns the store, prompts, and picker and feeds the machine
navigation-lifecycle events.

Also make PasswordManagerDataSourceProvider.requestAuthenticationIfNeeded
report failure (instead of dropping the completion) when local
authentication cannot be evaluated, so callers no longer hang.