Optimize directory traversal to skip excluded directories during enumeration

[Copilot]

WorkspaceTypeDiscovery.SearchDirectoryForTypeAsync was recursing into bin, obj, .git, and node_modules directories then filtering their files individually, causing unnecessary filesystem traversal.

Changes:

• Implemented manual recursion in SearchDirectoryRecursiveAsync that checks directory names before descending, avoiding enumeration of excluded directories entirely
• Moved excluded directories to a static readonly HashSet<string> to eliminate repeated allocations on each recursive call

Before:

var options = new EnumerationOptions { RecurseSubdirectories = true, IgnoreInaccessible = true };
foreach (var file in Directory.EnumerateFiles(directory, "*.cs", options))
{
    var pathSegments = fullPath.Split(Path.DirectorySeparatorChar, Path.AltDirectorySeparatorChar);
    if (pathSegments.Any(s => s is "bin" or "obj" or ".git" or "node_modules")) continue;
    // ...
}

After:

foreach (var subDir in Directory.EnumerateDirectories(directory, "*", new EnumerationOptions { IgnoreInaccessible = true }))
{
    if (ExcludedDirectories.Contains(Path.GetFileName(subDir))) continue;
    var result = await SearchDirectoryRecursiveAsync(subDir, typeName);
    // ...
}

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[Copilot]

Pull request overview

This PR optimizes workspace type discovery by avoiding unnecessary traversal into common non-source directories (e.g., bin/obj/.git/node_modules) and centralizes the exclusion logic. The core behavior of finding type-definition files remains the same, but with reduced filesystem overhead and cleaner directory filtering.

Changes:

• Introduced a static ExcludedDirectories HashSet<string> with case-insensitive comparison to centralize and reuse directory-exclusion logic.
• Replaced EnumerationOptions.RecurseSubdirectories usage with an explicit async recursive helper SearchDirectoryRecursiveAsync, which skips excluded directories before descending.
• Adjusted file processing to use the path provided by Directory.EnumerateFiles directly (instead of Path.GetFullPath) while preserving existing Roslyn-based verification.

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