Allow string service tiers in config TOML

[aibrahim-oai]

Why

service_tier in config.toml and profile config was still modeled as an enum, which blocked newer or experimental service tier IDs even though the runtime paths already carry string IDs.

This change makes the TOML-facing config accept string service tier IDs directly while keeping the legacy fast alias behavior by normalizing it to the request value priority.

What Changed

• change the TOML-facing service_tier fields in global and profile config to Option<String>
• keep config-load normalization so legacy fast still resolves to priority
• persist resolved service tier strings directly in config locks so arbitrary IDs round-trip cleanly
• regenerate the config schema and add config coverage for arbitrary string IDs plus legacy fast normalization

Verification

• added config tests for arbitrary string service tiers and legacy fast normalization
• ran just write-config-schema
• CI