Remove OnPrepareDetectionAsync method from the DockerfileComponentDetector

[Julian (jpinz)]

This pull request simplifies the DockerfileComponentDetector by removing unused code and dependencies related to skipped folder handling. The filtering logic for skipped folders has been removed, likely because it is now handled elsewhere or is no longer needed.

Code cleanup and simplification:

• Removed the OnPrepareDetectionAsync override and the associated logic that filtered out files in skipped folders, along with the private IsInSkippedFolder helper method. (src/Microsoft.ComponentDetection.Detectors/dockerfile/DockerfileComponentDetector.cs) [1] [2]
• Removed unused System.Linq and System.Reactive.Linq imports. (src/Microsoft.ComponentDetection.Detectors/dockerfile/DockerfileComponentDetector.cs)

[Copilot]

Copilot review overview

🟡 Changes recommended

Removing the filter regresses skipped-folder behavior and breaks an existing detector test.

Review effort: Balanced
Findings: 1

Open (1)

• Preserve SkippedFolders filtering for Dockerfile detection · New

What changed in this PR

Simplifies Dockerfile detection by removing detector-local skipped-folder filtering.

Changes:

• Removes OnPrepareDetectionAsync and its helper.
• Removes unused reactive/LINQ imports.

File	Description
DockerfileComponentDetector.cs	Removes node_modules request filtering.

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[github-actions]

👋 Hi! It looks like you modified some files in the Detectors folder.
You may need to bump the detector versions if any of the following scenarios apply:

• The detector detects more or fewer components than before
• The detector generates different parent/child graph relationships than before
• The detector generates different devDependencies values than before

If none of the above scenarios apply, feel free to ignore this comment 🙂

[Copilot]

Copilot review overview

🟡 Changes recommended

Path matching regresses existing behavior, and the broad debug-log assertion will fail because two debug messages are emitted.

Review effort: Balanced
Findings: 1 · 1

Open (2)

• Restrict logger verification to parse-failure message · New
• Preserve case-insensitive path-segment matching · New

Resolved since last review (1)

• Preserve SkippedFolders filtering for Dockerfile detection