[林SO (Linxiushen)]

Why are these changes needed?

BaseGroupChat.run() and run_stream() advertise Sequence[BaseChatMessage], but task validation only accepts lists. A tuple or collections.UserList of valid messages raises ValueError before the team runs.

Accept sequences in the existing validation branch and update both method docstrings. Parameterize the existing group-chat test over lists, tuples and UserList in both embedded and supplied runtimes. The test covers run(), a streamed rerun, message order, unchanged inputs, and empty/invalid-element rejection.

Related issue number

Fixes #8422

Validation

• Unmodified base d07c667a3239ea4ab7af62a53b0f4ff205a1f23d: the new tuple/UserList cases fail in both runtimes; the two list controls pass (4 failed, 2 passed).
• Fixed focused regressions: 6 passed. Entire test_group_chat.py: 91 passed on Windows, Python 3.12.14.
• Workspace poe format --check and poe lint --no-fix pass. Mypy checks all 64 agentchat source/test files successfully using the package task's equivalent command with its config path quoted correctly; the Poe package wrapper splits this workspace's path at a space. Pyright on the two changed files and git diff --check also pass.
• poe docs-build succeeds with 34 warnings, including missing optional extension dependencies. Notebook execution is disabled by the repository configuration.
• Workspace poe mypy was attempted and stops in the unchanged agbench package with four errors involving missing stubs and Docker-related types. The full workspace check has not passed.

Core/agentchat were installed using frozen uv sync, with selected test/documentation dependencies added separately (the final installed package inventory is recorded). The 91 group-chat tests also pass after those additions. Full workspace/all-extras tests were not run. Tests use local echo/replay fixtures; no external model service was called.

AI assistance was used to prepare the change and run the recorded local checks.

Checks

• I've included the relevant method docstring changes.
• I've added tests corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.