.NET: Make all AgentSkillsProvider tools require approval by default and add auto-approval rules

[SergeyMenshykh]

Summary

Make all functions in AgentSkillsProvider require approval by default and allow configuring auto-approval via ToolApprovalAgentOptions.

Changes

• Wrap all three tools (load_skill, read_skill_resource, run_skill_script) with ApprovalRequiredAIFunction unconditionally
• Add ReadOnlyToolsAutoApprovalRule and AllToolsAutoApprovalRule static properties (following the FileAccessProvider pattern)
• Remove ScriptApproval property from AgentSkillsProviderOptions and UseScriptApproval from the builder
• Add sample showing auto-approval configuration via UseToolApproval