2- Use string service tiers in session protocol (#20971)

## Summary
- break service tier session/op/app-server protocol fields from the
closed enum to string tier ids
- send the service tier string directly through model requests, prewarm,
compaction, memories, and TUI/app-server turn starts
- regenerate app-server protocol JSON/TypeScript schemas, removing the
standalone ServiceTier TS enum

## Verification
- just fmt
- cargo check -p codex-core -p codex-app-server -p codex-tui
- just write-app-server-schema

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Co-authored-by: Codex <noreply@openai.com>

codex-rs/app-server/src/request_processors/thread_processor.rs

@@ -1188,7 +1188,7 @@ impl ThreadRequestProcessor {
         &self,
         model: Option<String>,
         model_provider: Option<String>,
-        service_tier: Option<Option<codex_protocol::config_types::ServiceTier>>,
+        service_tier: Option<Option<String>>,
         cwd: Option<String>,
         approval_policy: Option<codex_app_server_protocol::AskForApproval>,
         approvals_reviewer: Option<codex_app_server_protocol::ApprovalsReviewer>,