fix: suppress agenthub_config default on direct chat-model construction (#825)

[radu-mocanu]

Summary

• direct construction of UiPathChat, UiPathChatOpenAI, UiPathAzureChatOpenAI, UiPathChatBedrock, UiPathChatBedrockConverse, UiPathChatAnthropicBedrock, UiPathChatGoogleGenerativeAI, UiPathChatVertex no longer sends X-UiPath-AgentHub-Config: agentsruntime
• factory path used by low-code agents is unchanged and keeps the upstream agentsruntime default
• UIPATH_AGENTHUB_CONFIG env var is honored when set explicitly

Why

Fixes #825

temporary client-side workaround to unblock local debug and external-token runs that hit 403 from agenthub governance after v0.10.x. proper fix will need to reside in uipath-llm-clients-python https://github.com/UiPath/uipath-llm-client-python/blob/35d4d31eeccad14eb7d6e690524e91c6b78624e0/src/uipath/llm_client/settings/platform/settings.py#L68

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==0.10.16.dev1008264452",

  # Any version from PR
  "uipath-langchain>=0.10.16.dev1008260000,<0.10.16.dev1008270000"
]

[[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>=0.10.16.dev1008260000,<0.10.16.dev1008270000",
]