test(config): two e2e routing tests stop racing KIGI_CODE_BASE_URL

`e2e_default_model_with_session_routes_to_proxy` and
`e2e_duplicate_model_field_both_entries_survive` assert the default
model's base URL. They were not serial and read `KIGI_CODE_BASE_URL`
through `EndpointsConfig`, while two serial tests (auth_method.rs,
models_fetch.rs) point that variable at wiremock. When the runs
overlapped, the base URL came back as the wiremock URI: the Linux suite
failed in 2 of 13 full runs. Presetting the variable fails both tests
every time, on v0.1.15 as well.

Both tests are now `#[serial]` and unset the variable for their scope,
the same cure `e2e_enterprise_endpoints_only_no_model_override` got for
the moonshot variable. With the variable preset they pass.

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

@@ -6707,7 +6707,10 @@ reasoning_effort = "low"
         assert_eq!(model.info.base_url, "https://inference.example.com/v1");
     }
     #[test]
+    #[serial]
     fn e2e_default_model_with_session_routes_to_proxy() {
+        // Hermetic: serial tests point KIGI_CODE_BASE_URL at wiremock.
+        let _base = EnvGuard::unset(kigi_env::CODE_BASE_URL_ENV);
         let (_, models) = resolve_models_from_toml("", None);
         let model = models
             .get(BUNDLED_DEFAULT_KEY)
@@ -6839,7 +6842,10 @@ reasoning_effort = "low"
         );
     }
     #[test]
+    #[serial]
     fn e2e_duplicate_model_field_both_entries_survive() {
+        // Hermetic: serial tests point KIGI_CODE_BASE_URL at wiremock.
+        let _base = EnvGuard::unset(kigi_env::CODE_BASE_URL_ENV);
         let dm = crate::models::default_model();
         let (_, models) = resolve_models_from_toml(
             &format!(