Add OpenAI platform: live model fetching with enrichment (provider 1)

The 4th registry row: id "openai", OPENAI_API_KEY env > auth.json
"openai" scope (login picker/paste/validation all registry-generic —
zero TUI changes needed, pinned by the picker test), base
https://api.openai.com/v1 with KIGI_OPENAI_BASE_URL override, Responses
dialect via the new PlatformWireApi spec field, enrichment-backed
metadata (wire_serves_metadata=false).

OpenAI's GET /v1/models returns bare ids and is polluted with
tts/whisper/embeddings entries: the listing is restricted to
enrichment-known TOOL-CALLING models (review caught that membership
alone admitted models.dev-known embeddings models, which would 400 on
every agentic request; dropped ids are debug-logged for launch-day
diagnosability). Context windows, effort menus, display names, and
thinking capability come from the enrichment pipeline — wiremock e2e
pins the full contract: polluted live listing + models.dev →
one Responses-backed chat model with a 400k documented context window.

Responses max-effort wiring (closes the P0c-1 debt): canonical effort
rides a CreateResponseWrapper sidecar and patch_reasoning_effort writes
it onto the serialized body at both send sites (all seven levels pinned,
xhigh/max distinct, summary preserved); normalize_effort_echo drops
echoes async-openai's typed enum cannot represent at both the non-stream
and SSE parse seams; the dead typed to_responses_api converter is
deleted. Kimi/moonshot stay byte-identical (ChatCompletions untouched,
wire_api maps to the same default; kimi wire tests green).

kimi-import now recognizes ANY registry platform host as built-in
(was hardcoded moonshot), covering openai and future rows.

crates/codegen/kigi-shell/src/agent/models.rs

@@ -141,6 +141,14 @@ impl PlatformApiKeys {
         }
         Self { keys }
     }
+
+    /// Test-only constructor for a single API-key platform.
+    #[cfg(test)]
+    pub(crate) fn test_single(platform: kigi_models::PlatformId, key: &str) -> Self {
+        Self {
+            keys: std::collections::BTreeMap::from([(platform, key.to_owned())]),
+        }
+    }
 }
 
 pub(crate) fn task_model_error_for_catalog(