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

… forward refs in docstrings, guard backend_client import in conftest

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

- config.py: enforcement mode no longer reads the UIPATH_GOVERNANCE_MODE env
  var directly (the backend /runtime/policy response is the source); default
  is AUDIT. Mode state lives in a holder object instead of a module global
  (no `global` statements). The test-only reset helper moved out to
  tests/_helpers.py so test concerns stay in the test tree.
- models.py: Check.logic is now the Logic(str, Enum) instead of a free-form
  string; tidy the CheckContext.model_name field.
- pyproject: add dependency upper bounds (pyyaml<7, vaderSentiment<4,
  chardet<8); remove the [[tool.mypy.overrides]] block.

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

EnforcementMode is a shared governance contract, so it now lives in
uipath.core.governance (uipath-core 0.5.19) and is re-exported from
config.py — runtime callers keep importing it from one place, but the
value type is owned by the lower abstraction level (per radu's review).
Bumps the uipath-core floor to 0.5.19.

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

feat(governance): enforcement-mode config, policy models, deps