Persist 'priority' service tier as fast in config

[aibrahim-oai]

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.

---

Codex Task

[etraut-openai]

Looks good