docs(.NET): Add pre-commit validation steps to CONTRIBUTING.md

[alzarei]

Add .NET Validation Steps to CONTRIBUTING.md

Motivation and Context

CONTRIBUTING.md currently provides minimal .NET validation guidance ("run build.cmd" or "bash build.sh"). Contributors need clear steps that match CI/CD requirements to catch issues locally before pushing.

Description

Adds comprehensive .NET validation steps to CONTRIBUTING.md with a 5-step pre-commit sequence:

1. Format Check: dotnet format --verify-no-changes
2. Build: dotnet build --warnaserror (matches CI/CD requirements)
3. Core Tests: Run ~1,500 SemanticKernel.UnitTests
4. AOT Check: Verify Native AOT compilation
5. Full Suite (optional): ~9,600+ tests

Includes:

• Both PowerShell and Bash commands for cross-platform support
• Output filtering to manage verbose build output
• Expected test counts and navigation instructions
• Quick validation shortcut for minimum requirements

Contribution Checklist

• The code builds clean without any errors or warnings (documentation-only change)
• The PR follows SK Contribution Guidelines
• All unit tests pass (N/A - docs only, markdown link check will run on PR)
• I didn't break anyone

Type of Change

• Documentation update