Fix MagenticOne handoff resume context reset and document expected HITL behavior (#7036)

[kowshik24]

Why are these changes needed?

MagenticOneGroupChat was not preserving context across human-in-the-loop handoff resumes.
After HandoffTermination(target="user"), a follow-up run()/run_stream(task=...) call was treated as a new task because
MagenticOneOrchestrator.handle_start rebuilt task/ledger state each time.

This PR fixes that behavior so resumes continue from preserved context (matching expected HITL semantics), adds a regression test,
and updates docs to clarify expected behavior.

What changed

• Fix in MagenticOneOrchestrator.handle_start:
  • Initialize task/facts/plan only on first run.
  • On subsequent runs, append user follow-up messages to existing thread.
  • Continue orchestration from preserved state instead of restarting planning.
• Added regression test:
  • test_magentic_one_handoff_resume_preserves_context
• Updated Human-in-the-Loop docs note:
  • Clarifies that MagenticOneGroupChat preserves context across handoff resumes.
  • Keeps existing Swarm-specific resume caveat.

Related issue number

Closes #7036

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

Local validation run:

• uv run --project python pytest -q packages/autogen-agentchat/tests/test_magentic_one_group_chat.py (passed)
• uv run --project python ruff check on modified src/test files (passed)

[kowshik24]

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