fix: switch BYOK model to gpt-5.3-codex

gpt-5.4 returns 400 on tools[5].function.name empty string,
likely a stricter validation in that model. Try gpt-5.3-codex.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: lpcox

.github/workflows/smoke-copilot-byok.lock.yml

@@ -38,7 +38,7 @@ safe-outputs:
 timeout-minutes: 5
 env:
   COPILOT_API_KEY: dummy-byok-key-for-offline-mode
-  COPILOT_MODEL: gpt-5.4
+  COPILOT_MODEL: gpt-5.3-codex
 features:
   cli-proxy: true
 strict: true