fix: explicitly exclude OAuth2 tests from CI pipeline

- Add oauth2 and requires_oauth tags to CI exclusions
- Ensures CI doesn't fail when .gemini_tokens file is not present
- These tags were already in always_excluded_tags but now explicitly in CI config

This prevents any OAuth2-related test failures in CI environments where
OAuth tokens cannot be configured.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

Author: azmaveth

lib/ex_llm/testing/config.ex

@@ -126,7 +126,9 @@ defmodule ExLLM.Testing.Config do
       external: true,
       live_api: true,
       requires_service: true,
-      local_only: true
+      local_only: true,
+      oauth2: true,
+      requires_oauth: true
     ]
   end
 

mix.exs

@@ -227,7 +227,7 @@ defmodule ExLLM.MixProject do
 
       # CI/CD pipeline tests
       "test.ci": [
-        "test --exclude wip --exclude flaky --exclude quota_sensitive --exclude very_slow --exclude requires_api_key --exclude integration --exclude external --exclude live_api --exclude requires_service --exclude local_only"
+        "test --exclude wip --exclude flaky --exclude quota_sensitive --exclude very_slow --exclude requires_api_key --exclude integration --exclude external --exclude live_api --exclude requires_service --exclude local_only --exclude oauth2 --exclude requires_oauth"
       ],
 
       # === CACHE MANAGEMENT ===