feat(providers): add ChatGPT/Codex subscription OAuth (PKCE + account-id, hardcoded catalog)

29th platform `openai-codex` (uses_oauth, Responses wire). PKCE-localhost login at
auth.openai.com (client app_EMoamEEZ73f0CkXaXp7hrann, redirect localhost:1455/auth/
callback, form token exchange, fresh-random state) reusing the claude-pro-max flow;
OAuthFlow::PkceLocalhost gained a redirect_path and OAuthConfig an authorize_extra
(empty elsewhere, so claude/xai/copilot authorize URLs stay byte-identical).

Codex-specific: the access token is a JWT carrying chatgpt_account_id, which becomes
the `chatgpt-account-id` inference header. It is derived STATELESSLY from whichever
bearer rides each request (so a rotated token needs no persisted field), and BOTH
login and refresh fail fast when the claim is absent — gated on the explicit
OAuthConfig.requires_chatgpt_account_id fact, never inferred from the token-body
encoding (a plain form endpoint is the OAuth norm and must not inherit this).

Inference rides the existing Responses wire at chatgpt.com/backend-api/codex →
/responses, with codex headers (chatgpt-account-id, originator, OpenAI-Beta
responses=experimental, codex UA) gated on SamplerConfig.openai_codex so API-key
`openai` stays byte-identical; store:false was already the global Responses default.

Catalog is HARDCODED (no live endpoint exists for this backend; read from the
official Codex CLI's model cache): gpt-5.6-sol/terra/luna + gpt-5.5, ctx 272000,
each with its real reasoning levels (low..ultra — ReasoningEffort gained Ultra).
Excluded: gpt-5.3-codex-spark (supported_in_api=false), gpt-5.4/-mini and
codex-auto-review (hidden) — they would list but fail at inference. The fetch
short-circuits before any HTTP; Kigi never shells out to the codex CLI or reads
~/.codex.

Security review fixes: redact any `account-id` header from request logs (it was
reaching debug logs), strict 3-segment JWT check (fail closed), refresh no longer
fails open on a missing claim. Inherits leak-safe pooled routing (scope
oauth/openai-codex) — never the Kimi token. Full gate green (234 suites, 0 warnings).

crates/codegen/kigi-shell/tests/common/mod.rs

@@ -40,6 +40,7 @@ pub fn test_sampler_config(
         auth_scheme: Default::default(),
         anthropic_oauth: false,
         github_copilot: false,
+        openai_codex: false,
         chat_compat: Default::default(),
         extra_headers: extra_headers
             .iter()