### Motivation
- Normalize persisted service tier so selecting the request value
`priority` (or legacy `fast`) is stored as `fast` while preserving
unknown tier IDs and keeping request-time behavior unchanged.
### Description
- Update persistence logic in `codex-rs/core/src/config/edit.rs` so
`ConfigEdit::SetServiceTier` maps request values: `priority`/`fast` ->
`"fast"`, `flex` -> `"flex"`, and leaves unknown strings unchanged.
- Add unit tests in `codex-rs/core/src/config/edit_tests.rs` that verify
a `priority` selection is written to `config.toml` as `"fast"` and that
unknown tiers are preserved.
- Add a config load test in `codex-rs/core/src/config/config_tests.rs`
to ensure `service_tier = "priority"` still resolves to the `priority`
request value at load time.
- Add the required import `use
codex_protocol::config_types::ServiceTier;` to the edited modules.
### Testing
- Ran `just fmt` and `just fix -p codex-core` to apply formatting and
lints and they completed successfully.
- Ran `cargo test -p codex-core --lib service_tier` (focused unit tests
for the change) and the tests passed.
- Ran `cargo test -p codex-protocol` and the protocol test suite passed.
- Note: an initial broader `cargo test -p codex-core service_tier`
invocation matched integration tests and produced unrelated
failures/hangs, so that run was interrupted and the focused `--lib`
unit-test invocation was used instead.
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[Codex
Task](https://chatgpt.com/codex/cloud/tasks/task_i_69ffc5a1262c8321af91b69c9845147f)
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