feat(models): gpt-6.1-sol joins the compiled-in codex catalog; codex User-Agent follows 0.159.0

The Codex backend (client_version 0.159.0) served eight list+supported models on
2026-09-29, gpt-6.1-sol first: context 272000, efforts low..max, default low.
POST /codex/responses with kigi's codex headers answers 200 and completes.

crates/codegen/kigi-sampling-types/src/types.rs

@@ -1202,7 +1202,7 @@ pub const CODEX_OPENAI_BETA: &str = "responses=experimental";
 /// openai-codex-gated). The Codex backend does not strictly validate the UA
 /// string (Pi ships its own and it works), so this is a stable best-effort
 /// identity, not a pinned build.
-pub const CODEX_USER_AGENT: &str = "codex_cli_rs/0.156.0";
+pub const CODEX_USER_AGENT: &str = "codex_cli_rs/0.159.0";
 /// JWT payload claim namespace carrying the ChatGPT account id.
 const CODEX_JWT_AUTH_CLAIM: &str = "https://api.openai.com/auth";