fix: preserve handoff target resolution compatibility in run-state agent maps

[seratch]

This pull request fixes #2421 handoff target discovery during run-state restoration by supporting both new handoff() weak-reference objects and legacy/custom handoff objects that expose .agent.

• Add a private _agent_ref weak reference to Handoff and assign it in handoff() so handoff-created targets can be traversed safely (src/agents/handoffs/init.py).
• Update _build_agent_map to resolve targets via agent_name, _agent_ref, and a backward-compatible .agent fallback, while skipping unresolved detached handoffs with debug logging (src/agents/run_state.py).
• Expand run-state tests to cover weakref-backed handoffs, legacy .agent handoffs, and unresolved handoff objects (tests/test_run_state.py).

[chatgpt-codex-connector]

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