Honor default service tier in Codex CLI

[shijie-oai]

Summary

• Thread backend-provided default_service_tier through Codex model metadata and resolve service_tier from config/model defaults
• Add local-only service_tier=default sentinel so opting out of Fast mode omits service_tier on inference requests
• Fall back to the model default service tier when a configured tier is unavailable for the selected model, and keep TUI Fast status based on the effective tier

Validation

• cargo test -p codex-protocol service_tier_resolution --lib
• cargo test -p codex-core get_service_tier --lib
• cargo test -p codex-tui user_turn_sends_standard_override_after_fast_is_turned_off --lib
• just fmt
• just fix -p codex-protocol -p codex-core -p codex-tui -p codex-models-manager -p codex-model-provider

Full workspace cargo test was not run locally because the scoped protocol/core/TUI coverage and clippy fix covered the touched behavior, and the full suite is intentionally large.