Add uipath_langchain_client, refactor code to use the new client

[cosminacho]

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==1.0.0.dev1006343357",

  # Any version from PR
  "uipath-langchain>=1.0.0.dev1006340000,<1.0.0.dev1006350000"
]

[[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>=1.0.0.dev1006340000,<1.0.0.dev1006350000",
]