feat: add A2A tool for invoking remote agents at runtime

[PopescuTudor]

Adds an A2A LangChain tool that enables low-code agents to invoke Remote A2A agents at runtime. When an agent definition includes an A2A resource ($resourceType: "a2a"), the tool factory now creates an A2aTool that sends messages through the AgentHub A2A proxy.

Two conversation persistence modes are supported:

• Graph state (advancedPersistance: true): task_id/context_id stored in LangGraph's tools_storage
• LLM passthrough (default): IDs included in tool schema so the LLM tracks conversation continuity

Uses the a2a-sdk Python client for A2A protocol communication, with lazy auth via UiPath Bearer token.

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==0.9.25.dev1007613980",

  # Any version from PR
  "uipath-langchain>=0.9.25.dev1007610000,<0.9.25.dev1007620000"
]

[[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>=0.9.25.dev1007610000,<0.9.25.dev1007620000",
]