Harden branding image symlink check to cover ancestor directories

[Mpdreamz]

Why

The previous symlink check on branding.icon and branding.og-image only tested whether the image file itself was a symlink. A symlinked parent directory (e.g. assets/ → /etc/) would pass the check and allow an attacker to read arbitrary files outside the documentation source directory.

However we still validate the file itself is an image.

What

Replaces the bare resolved.LinkTarget is not null check with ValidateFileAccess, which walks ancestor directories up to the doc root and rejects any symlinked or hidden intermediate directory — the same protection already used for docset.yml and toc.yml.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Enterprise

Run ID: 05606a7a-baf3-4686-8c1f-f01b08a0a5c6

📥 Commits

Reviewing files that changed from the base of the PR and between dea6859 and 63f3bbf.

📒 Files selected for processing (1)

• src/Elastic.Documentation.Configuration/Builder/ConfigurationFile.cs

✨ Finishing Touches

✨ Simplify code

• Create PR with simplified code
• Commit simplified code in branch fix/branding-symlink-ancestor-dirs

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