fix(provider): address Astrai follow-up review from #486

- Add "astrai" to factory_all_providers_create_successfully test
- Add "astrai" => "ASTRAI_API_KEY" in provider_env_var() for onboarding
- Add Astrai to onboarding provider selection list (Gateway tier)
- Add provider_env_var("astrai") assertion in known_providers test

Addresses review comments from @chumyin on #486.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/providers/mod.rs

@@ -1347,6 +1347,7 @@ mod tests {
             "cohere",
             "copilot",
             "nvidia",
+            "astrai",
         ];
         for name in providers {
             assert!(