fix: use origin identity check for types.UnionType in _unwrap_optional

[radu-mocanu]

Summary

• replace isinstance(annotation, types.UnionType) with origin is types.UnionType in _unwrap_optional
• add regression test for X | None (PEP 604) field annotations

Why

the previous fix used isinstance on the annotation object directly, which runs on every non-generic annotation (when get_origin returns None) and could raise on pathological annotations. checking origin is types.UnionType is equivalent, safer, and consistent with how the typing.Union case is already handled.

Development Package

• Use uipath pack --nolock to get the latest dev build from this PR (requires version range).
• Add this package as a dependency in your pyproject.toml:

[project]
dependencies = [
  # Exact version:
  "uipath-langchain==0.10.4.dev1008014385",

  # Any version from PR
  "uipath-langchain>=0.10.4.dev1008010000,<0.10.4.dev1008020000"
]

[[tool.uv.index]]
name = "testpypi"
url = "https://test.pypi.org/simple/"
publish-url = "https://test.pypi.org/legacy/"
explicit = true

[tool.uv.sources]
uipath-langchain = { index = "testpypi" }

[tool.uv]
override-dependencies = [
    "uipath-langchain>=0.10.4.dev1008010000,<0.10.4.dev1008020000",
]