feat: add InitializeFromRealDirectory

[vbreuss]

Adds support for initializing a MockFileSystem directory structure from an existing directory on the real file system, enabling easier seeding of mock data in tests.

Key Changes:

• Introduced InitializeFromRealDirectory on FileSystemInitializerExtensions to copy a real directory into an IFileSystem.
• Added unit tests covering file copy, recursive directory copy, and the missing-source-directory error case.

---

• Fixes Map actual directories in MockFileSystem #877

[Copilot]

Pull request overview

Adds support for initializing a MockFileSystem directory structure from an existing directory on the real file system, enabling easier seeding of mock data in tests.

Changes:

• Introduced InitializeFromRealDirectory on FileSystemInitializerExtensions to copy a real directory into an IFileSystem.
• Added unit tests covering file copy, recursive directory copy, and the missing-source-directory error case.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 6 comments.

File	Description
Source/Testably.Abstractions.Testing/FileSystemInitializerExtensions.cs	Adds the new initialization API and recursive copy implementation.
Tests/Testably.Abstractions.Testing.Tests/FileSystemInitializerExtensionsTests.cs	Adds test coverage for the new initializer behavior.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-actions]

Test Results

     52 files  ± 0       52 suites  ±0   37m 32s ⏱️ + 1m 5s
 64 501 tests +15   60 878 ✅ + 7   3 621 💤 +6  2 ❌ +2 
133 854 runs  +40  119 302 ✅ +32  14 550 💤 +6  2 ❌ +2 

For more details on these failures, see this check.

Results for commit 2eb09c0. ± Comparison against base commit 1253c6b.

[github-actions]

This is addressed in release v5.3.0.