Centralize file path validation for issue providers

[Copilot]

This PR centralizes the file path validation logic that was previously duplicated across multiple issue providers (MsBuild, TAP, and SARIF) into a single, reusable method on the BaseIssueProvider class.

Problem

The ValidateFilePath method existed in three different implementations with similar but slightly different logic:

• MsBuild: BaseMsBuildLogFileFormat.ValidateFilePath - validated absolute paths only
• TAP: BaseTapLogFileFormat.ValidateFilePath - handled both relative and absolute paths with auto-detection
• SARIF: SarifIssuesProvider.ValidateFilePath - took an explicit isAbsolutePath parameter

This duplication made the codebase harder to maintain and could lead to inconsistent behavior across providers.

Solution

Added a centralized BaseIssueProvider.ValidateFilePath static method that:

• Automatically detects if a path is relative or absolute (no need for manual detection)
• Validates that absolute paths are within the repository using existing extension methods
• Converts absolute paths to relative paths using the repository root
• Preserves relative paths unchanged
• Returns a consistent tuple format: (bool Valid, string FilePath)

All existing provider implementations now delegate to this centralized method:

protected (bool Valid, string FilePath) ValidateFilePath(string filePath, IRepositorySettings repositorySettings) =>
    BaseIssueProvider.ValidateFilePath(filePath, repositorySettings);

Benefits

• Eliminates code duplication: Removed ~60 lines of duplicated validation logic
• Ensures consistency: All issue providers now use identical validation behavior
• Improves maintainability: Changes to validation logic only need to be made in one place
• Comprehensive coverage: Added thorough tests for all validation scenarios

Testing

• All existing tests continue to pass (no regressions)
• Added comprehensive test coverage for the new centralized method
• Verified functionality across MsBuild, TAP, and SARIF providers

Fixes #975.

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