Add custom exception messages for PSGallery

[shammu1]

PR Summary

This pull request enhances error handling and diagnostics when publishing packages to the PowerShell Gallery (PSGallery), making it easier to identify and troubleshoot authentication and permission issues.

PR Context

This PR is needed for PSResourceGET to provide the users the exceptions messages with exact reason phrase for PSGallery.

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• Make sure all .h, .cpp, .cs, .ps1 and .psm1 files have the correct copyright header
• This PR is ready to merge and is not Work in Progress.
  • If the PR is work in progress, please add the prefix WIP: or [ WIP ] to the beginning of the title (the WIP bot will keep its status check at Pending while the prefix is present) and remove the prefix when the PR is ready.
• Breaking changes
  • None
  • OR
  • Documentation needed
    • Issue filed:
• User-facing changes
  • Not Applicable
  • OR
  • Documentation needed
    • Issue filed:
• Testing - New and feature
  • N/A or can only be tested interactively
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  • Make sure you've added a new test if existing tests do not effectively test the code changed
• Tooling
  • I have considered the user experience from a tooling perspective and don't believe tooling will be impacted.
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🟡 Changes recommended

Add coverage validating the new PSGallery diagnostic messages.

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Pull request overview

This PR improves PSGallery publishing diagnostics with clearer authentication, permission, and duplicate-package errors.

Changes:

• Exposes the PSGallery URI.
• Extracts package identity from .nupkg files.
• Adds custom handling for HTTP 401, 403, and 409 responses.

File summaries

File	Summary	Findings
src/code/RepositorySettings.cs	Exposes the PSGallery URI constant.	None
src/code/PublishHelper.cs	Adds package identity extraction and PSGallery-specific diagnostics.	Moderate: add tests covering the exact 401, 403, and 409 messages, including duplicate-package formatting.

Review details

Suppressed comments (1)

src/code/PublishHelper.cs:749

• This branch is selected for 401 Unauthorized, but the message hard-codes the unrelated An API key must be provided reason. The preceding Contains("API") branch already handles the missing-key response, so a bad credential is currently reported with the wrong cause; preserve e.Message (or use the actual Unauthorized reason) here.

                        error = new ErrorRecord(new ArgumentException($"Could not publish to repository '{repoName}'. The Credential provided was incorrect. Exception: Response status code does not indicate success: 401 (An API key must be provided)."),

• Files reviewed: 2/2 changed files
• Comments generated: 1
• Review effort level: Lite

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