Fix bugs, improve error handling, update packages, and add unit tests

[Steveiwonder]

• Fix InMemoryFakeDataSource.GetCount() returning table count instead of row count
• Fix SqlDataSource.GetData() connection leak (missing using statement)
• Fix race condition in unique value tracking (HashSet not thread-safe)
• Fix GetUniqueValue double-check pattern replaced with GetOrAdd + lock
• Make BogusDataProvider._supported static readonly
• Rename SqlDataPovider.cs to SqlDataProvider.cs (typo fix)
• Replace generic Exception throws with InvalidOperationException throughout
• Add File.Exists checks with descriptive errors in Config.Load()
• Add null/empty guard for SourceColumns in MaskComputed
• Update all NuGet packages to latest versions
• Add 65 unit tests covering BogusDataProvider, Extensions, Utils, DataSourceProvider, DataMasker, and InMemoryFakeDataSource
• Add UseList feature to ColumnConfig for random selection from a predefined list
• Add InternalsVisibleTo for DataMasker.Tests

[gitguardian]

⚠️ GitGuardian has uncovered 1 secret following the scan of your pull request.

Please consider investigating the findings and remediating the incidents. Failure to do so may lead to compromising the associated services or software components.

🔎 Detected hardcoded secret in your pull request

GitGuardian id	GitGuardian status	Secret	Commit	Filename
30524701	Triggered	Generic Password	cd3d9df	src/DataMasker.Tests/DataSources/SqlDataSourceTests.cs	View secret

🛠 Guidelines to remediate hardcoded secrets

1. Understand the implications of revoking this secret by investigating where it is used in your code.
2. Replace and store your secret safely. Learn here the best practices.
3. Revoke and rotate this secret.
4. If possible, rewrite git history. Rewriting git history is not a trivial act. You might completely break other contributing developers' workflow and you risk accidentally deleting legitimate data.

To avoid such incidents in the future consider

• following these best practices for managing and storing secrets including API keys and other credentials
• install secret detection on pre-commit to catch secret before it leaves your machine and ease remediation.

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