Replace subprocess tests with in-process async servers (#2006)

* Use anyio as testing backend

* Remove asyncio markers

* Update streamable http tests

* Replace all subprocess tests

* Replace anyio task groups with asyncio context managers in tests

- Convert run_server_async from anyio task group pattern to asyncio.create_task with async context manager
- Remove task_group fixture from conftest
- Update all test fixtures to use async with run_server_async pattern
- Remove TaskGroup imports from all test files
- Tests now work with pytest-asyncio instead of pytest-anyio

* Update test_github_provider_integration.py

tests/server/auth/providers/test_github.py

@@ -177,7 +177,6 @@ class TestGitHubTokenVerifier:
         )  # Parent TokenVerifier sets empty list as default
         assert verifier.timeout_seconds == 10
 
-    @pytest.mark.asyncio
     async def test_verify_token_github_api_failure(self):
         """Test token verification when GitHub API returns error."""
         verifier = GitHubTokenVerifier()
@@ -196,7 +195,6 @@ class TestGitHubTokenVerifier:
             result = await verifier.verify_token("invalid_token")
             assert result is None
 
-    @pytest.mark.asyncio
     async def test_verify_token_success(self):
         """Test successful token verification."""
         from unittest.mock import AsyncMock