.NET: Support reflection for discovery of resources and scripts in class-based skills

[SergeyMenshykh]

This PR adds support for using reflection to discover resources and scripts in class-based skills, enabling a more declarative approach via attributes.

Changes

• Add AgentSkillResourceAttribute and AgentSkillScriptAttribute for declarative resource/script discovery
• Enhance AgentClassSkill to discover resources and scripts via reflection
• Update samples to demonstrate the new attribute-based approach

[Copilot]

Pull request overview

Adds attribute-based reflection discovery for resources and scripts in class-based .NET skills, enabling a more declarative model while keeping existing override-based patterns available.

Changes:

• Introduces [AgentSkillResource] and [AgentSkillScript] attributes for declarative discovery.
• Updates AgentClassSkill to a CRTP generic (AgentClassSkill<TSelf>) and implements reflection-based discovery + caching.
• Updates unit tests and samples to use the new generic base class and attribute-based approach (including DI scenarios).

Reviewed changes

Copilot reviewed 19 out of 19 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
dotnet/src/Microsoft.Agents.AI/Skills/Programmatic/AgentClassSkill.cs	Converts to AgentClassSkill<TSelf>, adds reflection discovery + caching, and adds SerializerOptions support.
dotnet/src/Microsoft.Agents.AI/Skills/Programmatic/AgentSkillResourceAttribute.cs	Adds attribute for marking properties/methods as discoverable resources.
dotnet/src/Microsoft.Agents.AI/Skills/Programmatic/AgentSkillScriptAttribute.cs	Adds attribute for marking methods as discoverable scripts.
dotnet/src/Microsoft.Agents.AI/Skills/Programmatic/AgentInlineSkillResource.cs	Adds MethodInfo-based constructor for reflected resource members.
dotnet/src/Microsoft.Agents.AI/Skills/Programmatic/AgentInlineSkillScript.cs	Adds MethodInfo-based constructor for reflected script members.
dotnet/src/Microsoft.Agents.AI/Skills/AgentSkillsProviderBuilder.cs	Updates docs to reference AgentClassSkill{TSelf}.
dotnet/tests/Microsoft.Agents.AI.UnitTests/AgentSkills/AgentClassSkillTests.cs	Expands coverage for reflection discovery, caching, DI, and serializer options behavior.
dotnet/tests/Microsoft.Agents.AI.UnitTests/AgentSkills/AgentSkillsProviderTests.cs	Adjusts tests to accommodate AgentClassSkill<TSelf> and mixed AgentSkill collections.
dotnet/tests/Microsoft.Agents.AI.UnitTests/AgentSkills/AgentSkillResourceAttributeTests.cs	Adds unit tests for AgentSkillResourceAttribute constructors.
dotnet/tests/Microsoft.Agents.AI.UnitTests/AgentSkills/AgentSkillScriptAttributeTests.cs	Adds unit tests for AgentSkillScriptAttribute constructors.
dotnet/tests/Microsoft.Agents.AI.UnitTests/AgentSkills/AgentInlineSkillResourceTests.cs	Adds tests for MethodInfo-based resource behavior.
dotnet/tests/Microsoft.Agents.AI.UnitTests/AgentSkills/AgentInlineSkillScriptTests.cs	Adds tests for MethodInfo-based script behavior and schema.
dotnet/samples/02-agents/AgentSkills/Agent_Step03_ClassBasedSkills/Program.cs	Updates class-based sample to attribute discovery + CRTP base class.
dotnet/samples/02-agents/AgentSkills/Agent_Step03_ClassBasedSkills/README.md	Updates documentation to describe attribute discovery usage.
dotnet/samples/02-agents/AgentSkills/Agent_Step03_ClassBasedSkills/Agent_Step03_ClassBasedSkills.csproj	Adds IDE warning suppression for attribute-only private members.
dotnet/samples/02-agents/AgentSkills/Agent_Step04_MixedSkills/Program.cs	Updates mixed-skills sample to attribute discovery + CRTP base class.
dotnet/samples/02-agents/AgentSkills/Agent_Step04_MixedSkills/Agent_Step04_MixedSkills.csproj	Adds IDE warning suppression for attribute-only private members.
dotnet/samples/02-agents/AgentSkills/Agent_Step05_SkillsWithDI/Program.cs	Updates DI sample to attribute discovery for resource/script methods.
dotnet/samples/02-agents/AgentSkills/Agent_Step05_SkillsWithDI/Agent_Step05_SkillsWithDI.csproj	Adds IDE warning suppression for attribute-only private members.