.NET: Sanitize agent name

[SergeyMenshykh]

Fixes #1351 for AF .NET.

[Copilot]

Pull Request Overview

This PR adds agent name sanitization functionality to ensure agent names are compatible when used as AI function names. The implementation replaces invalid characters with underscores to create valid function identifiers.

• Sanitizes agent names by replacing non-alphanumeric characters (except underscores) with underscores
• Uses source-generated regex for .NET and compiled regex for older frameworks for optimal performance
• Adds comprehensive test coverage for various agent name patterns and edge cases

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
dotnet/src/Microsoft.Agents.AI/AgentExtensions.cs	Implements agent name sanitization logic with regex-based character replacement
dotnet/tests/Microsoft.Agents.AI.UnitTests/AgentExtensionsTests.cs	Adds parameterized tests to verify sanitization behavior for different input patterns

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