fix: prevent cross-provider model env var leaks and sync Codex detection (#243)

Two provider routing bugs that cause silent wrong-model failures:

1. model.ts: getUserSpecifiedModelSetting() read ANTHROPIC_MODEL ||
   GEMINI_MODEL || OPENAI_MODEL with no provider check. A user
   switching from Anthropic to OpenAI with ANTHROPIC_MODEL still set
   would silently send the Anthropic model name to the OpenAI API.
   Now gates each env var behind the active provider from
   getAPIProvider().

2. providers.ts: isCodexModel() maintained a hardcoded list of 8 model
   names that was missing gpt-5.4-mini and gpt-5.2 from the canonical
   CODEX_ALIAS_MODELS table in providerConfig.ts. This caused a
   split-brain: getAPIProvider() returned 'openai' while
   resolveProviderRequest() selected 'codex_responses' transport.
   Now delegates to the exported isCodexAlias() to keep both detection
   systems in sync.

src/services/api/providerConfig.ts

@@ -201,7 +201,7 @@ function parseModelDescriptor(model: string): ModelDescriptor {
   }
 }
 
-function isCodexAlias(model: string): boolean {
+export function isCodexAlias(model: string): boolean {
   const normalized = model.trim().toLowerCase()
   const base = normalized.split('?', 1)[0] ?? normalized
   return base in CODEX_ALIAS_MODELS