.NET: Fix YAML block scalar parsing for file skills

[tejakusireddy]

Motivation and Context

Fixes the .NET file skill loader portion of #5586.

SKILL.md frontmatter currently parses YAML block scalar values like description: | or description: > as only the scalar indicator (| / >) instead of the multiline content.

Description

This PR updates the .NET file skill loader to parse YAML block scalar values in frontmatter.

Changes:

• Supports literal scalars: |, |-, |+
• Supports folded scalars: >, >-, >+
• Strips common indentation from block content
• Preserves newlines for literal scalars
• Folds lines with spaces for folded scalars
• Adds regression coverage in FileAgentSkillLoaderTests

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contributing Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? No

Validation:

• dotnet build ./dotnet/tests/Microsoft.Agents.AI.UnitTests/Microsoft.Agents.AI.UnitTests.csproj
• FileAgentSkillLoaderTests: 79/79 passed
• Microsoft.Agents.AI.UnitTests net10.0 assembly: 1459/1459 passed

[Copilot]

Pull request overview

This PR fixes .NET SKILL.md frontmatter parsing so YAML block scalar values (e.g., description: | / description: >) are parsed as multiline content instead of just the scalar indicator, addressing issue #5586.

Changes:

• Updates the .NET file skill loader to detect block scalar indicators and read indented continuation lines.
• Normalizes indentation and applies basic literal (|) vs folded (>) joining behavior.
• Adds unit tests covering literal, folded, and chomping-indicator variants.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
dotnet/src/Microsoft.Agents.AI/Skills/File/AgentFileSkillsSource.cs	Adds block-scalar-aware parsing when extracting frontmatter scalar values.
dotnet/tests/Microsoft.Agents.AI.UnitTests/AgentSkills/FileAgentSkillLoaderTests.cs	Adds regression tests for parsing multiline description values from YAML block scalars.