fix: Remove `auth`'s `expect()` usage (#12895)
## Why
`turborepo-auth` still required a crate-level `.expect()` allowance for
a few implementation paths. That kept auth outside the workspace
panic-hardening policy even after the unwrap cleanup.
Closes TURBO-5602
## What
Removes the remaining implementation `.expect()` calls from auth time
handling and SSO notification redirect URL parsing, then drops the
crate-level `clippy::expect_used` allowance.
## How
- `cargo fmt -p turborepo-auth`
- `cargo test -p turborepo-auth`
- `cargo clippy -p turborepo-auth --all-targets -- -D warnings`
- Pre-push hook passed with format, check:toml, and Rust checks
