test: address claude-review on Pact consumer contract (#883)

- pact.yml: pin tailscale/github-action@v3 to commit SHA (3 jobs) and
  pact-ruby-standalone install.sh to v2.6.1 (2 jobs) — supply-chain hardening
- pact.yml: drop redundant `-o "addopts=..."` override (pyproject.toml already
  sets the same addopts; the override would silently mask future additions)
- test_mcp_provider_verification.py: remove dead `pytestmark` shadowed by the
  list assignment; gate the module skip on PACT_USERNAME/PACT_PASSWORD too so a
  broker-set-but-creds-missing CI skips cleanly instead of raising KeyError
- conftest.py: drop the unused `pact_dir` fixture

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/contract/conftest.py

@@ -30,12 +30,6 @@ def _clean_pact_dir():
     yield
 
 
-@pytest.fixture
-def pact_dir() -> Path:
-    """Directory the generated pact files are written to."""
-    return PACT_DIR
-
-
 @pytest.fixture
 def consumer_pact():
     """A fresh Pact (consumer=nextcloud-mcp-server, provider=astrolabe).