.NET: Fix handoff function naming

[stephentoub]

We don't need the agent's name or a guid in the handoff name... we can just use simple numbering. There's a possibility that someone built an agent with a built-in function tool named "handoff_to_x"; if that turns out to be an issue, we could add back some longer bit of randomness.

Fixes #1344

[Copilot]

Pull Request Overview

This PR simplifies handoff function naming by using simple incremental numbering instead of agent names or GUIDs. This change addresses potential conflicts where agents might have built-in function tools with similar names.

• Replaces descriptive agent-based naming with simple numeric indexing
• Introduces an index counter that increments for each handoff target
• Maintains the same function prefix while using cleaner numbering scheme

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