Fix indexed icon media resource resolution

[arista-ms]

Summary of the Pull Request

Fixes indexed .exe, .dll, and .lnk profile icons being rejected during media resource validation.

References and Relevant Issues

Closes #20567

Regression from #19143. Restores the indexed icon behavior introduced by #14107 / #1504.

Detailed Description of the Pull Request / Additional comments

Media resource validation currently checks the complete icon value, including its ,index suffix, with std::filesystem::exists. For example, it checks for a file literally named shell32.dll,210, rejects the resource, and prevents IconPathConverter from extracting the requested icon.

This change:

• recognizes signed icon indexes for .exe, .dll, and .lnk resources;
• validates and resolves the binary path without the index;
• preserves the original index for IconPathConverter;
• splits on the final comma so commas in directory and file names remain valid;
• leaves non-icon media resources under strict whole-path validation.

Validation Steps Performed

• Built the full Debug x64 solution with the repository's standard Invoke-OpenConsoleBuild workflow.
• Ran SettingsModelUnitTests::MediaResourceTests::RealResolverIndexedIconPaths.
• Verified absolute, relative, environment-expanded, positive, negative, zero, malformed, and non-icon indexed paths.
• Verified paths with commas in both directory names and file names, with and without an index.
• Registered and launched the local Windows Terminal Dev package and visually confirmed an icon loaded from oicons.dll,24.

PR Checklist

• Closes Indexed DLL/EXE profile icons are rejected by media resource validation #20567
• Tests added/passed
• Documentation updated
  • Not necessary; this restores previously supported behavior and does not change the settings schema.
• Schema updated (if necessary)
  • Not necessary.

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