Move WinML EP init mock to root conftest for global coverage

[timenick]

Summary

• Move autouse fixture _skip_winml_ep_init from tests/session/conftest.py to root tests/conftest.py
• winml.modelkit.models.winml.base imports WinMLSession at module level, which triggers WinMLEPRegistry._discover_eps() → WinML SDK runtime initialization on any winml.modelkit import
• This causes ALL test groups (unit, models, commands, etc.) to potentially hang on CI environments without the SDK installed — not just tests/session
• The fixture mocks WinMLSession._init_winml_eps_once for non-e2e tests; e2e tests (marked @pytest.mark.e2e) bypass the mock and use real SDK initialization

Import chain

winml.modelkit.__init__
  → winml.modelkit.models
    → winml.modelkit.models.winml
      → winml.modelkit.models.winml.base
        → winml.modelkit.session.session (WinMLSession)
          → winml.modelkit.session.ep_registry (WinMLEPRegistry)