fix: prevent ANTHROPIC_API_KEY from interfering with Gemini provider auth

Two fixes for issue #133 where setting ANTHROPIC_API_KEY=dummy alongside
CLAUDE_CODE_USE_GEMINI=1 causes "Invalid API key" errors:

1. auth.ts: In the CI branch of getAnthropicApiKeyWithSource(), the
   ANTHROPIC_API_KEY value was returned without checking isUsing3PServices().
   A dummy key leaked into the Anthropic key resolution pipeline even when
   Gemini was the active provider. Now guards with isUsing3PServices().

2. errors.ts: The x-api-key error handler surfaced "Invalid API key" for
   any provider. Added getAPIProvider() === 'firstParty' guard so 3P users
   see the real underlying error instead of a misleading auth message.

Note: The cli.tsx Gemini validation fix (originally part of this PR) was
independently implemented in PR #121 and is already on main.

src/utils/auth.ts

@@ -286,7 +286,7 @@ export function getAnthropicApiKeyWithSource(
       )
     }
 
-    if (apiKeyEnv) {
+    if (apiKeyEnv && !isUsing3PServices()) {
       return {
         key: apiKeyEnv,
         source: 'ANTHROPIC_API_KEY',
@@ -294,6 +294,7 @@ export function getAnthropicApiKeyWithSource(
     }
 
     // OAuth token is present but this function returns API keys only
+    // Also reached when 3P provider is active — ANTHROPIC_API_KEY is ignored
     return {
       key: null,
       source: 'none',