(#3191) Pass custom headers to web metadata requests

[wtg-mg8]

Description Of Changes

Forward Options.Headers from Get-ChocolateyWebFile to Get-WebHeaders and Get-WebFileName so authenticated endpoints work during probing.

Add a Pester regression test covering metadata and download requests.

GPT5.6 generated, human reviewed.

Motivation and Context

See issue #3919 (Duplicate of #3191)

Testing

Tested locally by executing the powershell test directly, functionally testing that it works against our own endpoint and running the entire pester suite.

Tested using Powershell v3 and Powershell v5.

Operating Systems Testing

Tested on Windows 11.

Change Types Made

• Bug fix (non-breaking change).
• Feature / Enhancement (non-breaking change).
• Breaking change (fix or feature that could cause existing functionality to change).
• Documentation changes.
• PowerShell code changes.

Change Checklist

• Requires a change to the documentation.
• Documentation has been updated.
• Tests to cover my changes, have been added.
• All new and existing tests passed?
• PowerShell code changes: PowerShell v3 compatibility checked? Changes to the exported functions are compatible according to syntax checking using Powershell v2. Test suite is not compatible, as it uses Pester v5 syntax, which needs Powershell v5.1.
• All items are complete on the Definition of Done.

Related Issue

Fixes #3191 (and duplicate that I originally raised: #3919)