fix: clear agenthub_config via field_validator so headers are correct (#825)

[radu-mocanu]

Summary

• re-exported chat models no longer leak X-UiPath-AgentHub-Config: agentsruntime on direct construction

Why

Fixes #825

the previous fix used a model_validator(mode="after") to null client_settings.agenthub_config which runs after upstream validators which materialize the inner httpx client during ctor and cache its headers from the still-default agenthub_config="agentsruntime".

moving to field_validator("client_settings", mode="after") clears the field before any model validator accesses the cached client.

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==0.10.18.dev1008304470",

  # Any version from PR
  "uipath-langchain>=0.10.18.dev1008300000,<0.10.18.dev1008310000"
]

[[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>=0.10.18.dev1008300000,<0.10.18.dev1008310000",
]