.NET: Add inline skills API

[SergeyMenshykh]

Summary

Adds the inline skills API — code-defined skills with resources (static values or delegates) and scripts (delegates).

New types

• AgentInlineSkill — Skill defined entirely in code with XML content generation, resources, and scripts
• AgentInlineSkillResource — Skill resource backed by a static value or delegate
• AgentInlineSkillScript — Skill script backed by a delegate with automatic parameter marshaling

Integration

• AgentSkillsProvider constructors accepting inline skills
• AgentSkillsProviderBuilder.UseInlineSkill(s) methods

Contributes to: #4350

[Copilot]

Pull request overview

Adds a new “inline skills” surface area to the .NET Agent Skills system, enabling code-defined skills with delegate-backed resources and scripts, plus builder/provider integration and accompanying tests/samples.

Changes:

• Introduces programmatic skill types: AgentInlineSkill, AgentInlineSkillResource, AgentInlineSkillScript.
• Adds inline-skill integration points in AgentSkillsProvider and AgentSkillsProviderBuilder (plus an internal AgentInMemorySkillsSource).
• Updates unit tests and adds a new sample demonstrating code-defined skills.

Reviewed changes

Copilot reviewed 19 out of 19 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
dotnet/src/Microsoft.Agents.AI/Skills/Programmatic/AgentInlineSkill.cs	New code-defined skill type that synthesizes skill content and registers resources/scripts.
dotnet/src/Microsoft.Agents.AI/Skills/Programmatic/AgentInlineSkillResource.cs	New resource implementation backed by a static value or delegate.
dotnet/src/Microsoft.Agents.AI/Skills/Programmatic/AgentInlineSkillScript.cs	New script implementation backed by a delegate via AIFunctionFactory.
dotnet/src/Microsoft.Agents.AI/Skills/AgentInMemorySkillsSource.cs	New internal skills source to serve in-memory AgentSkill instances.
dotnet/src/Microsoft.Agents.AI/Skills/AgentSkillsProvider.cs	Adds provider constructors accepting inline skills; adjusts file-based constructor behavior.
dotnet/src/Microsoft.Agents.AI/Skills/AgentSkillsProviderBuilder.cs	Adds UseInlineSkill(s) APIs for composing inline skills into a provider.
dotnet/src/Microsoft.Agents.AI/Skills/AgentSkill.cs	Updates documentation to mention inline skills and synthesized content.
dotnet/tests/Microsoft.Agents.AI.UnitTests/AgentSkills/AgentInlineSkillTests.cs	New unit tests for inline skill frontmatter/content/resource/script behavior.
dotnet/tests/Microsoft.Agents.AI.UnitTests/AgentSkills/AgentInlineSkillResourceTests.cs	New unit tests for inline resource behavior.
dotnet/tests/Microsoft.Agents.AI.UnitTests/AgentSkills/AgentInlineSkillScriptTests.cs	New unit tests for inline script delegate invocation and schema.
dotnet/tests/Microsoft.Agents.AI.UnitTests/AgentSkills/AgentInMemorySkillsSourceTests.cs	New unit tests for in-memory skills source.
dotnet/tests/Microsoft.Agents.AI.UnitTests/AgentSkills/AgentSkillsProviderTests.cs	Adds tests exercising mixed sources / inline constructor usage.
dotnet/tests/Microsoft.Agents.AI.UnitTests/AgentSkills/FilteringAgentSkillsSourceTests.cs	Updates tests to use AgentInMemorySkillsSource + inline skills.
dotnet/tests/Microsoft.Agents.AI.UnitTests/AgentSkills/DeduplicatingAgentSkillsSourceTests.cs	Updates tests to use inline skills / in-memory source.
dotnet/samples/02-agents/AgentSkills/README.md	Documents file-based vs code-defined skills and links new sample.
dotnet/samples/02-agents/AgentSkills/Agent_Step02_CodeDefinedSkills/*	New sample demonstrating inline skills with resources and scripts.
dotnet/agent-framework-dotnet.slnx	Adds the new sample project; also removes some unrelated projects/solution items.