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
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• 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.
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• 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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[Copilot]

🟡 Changes recommended

Address the moderate diagnostics, test coverage, and URI normalization findings before approval.

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Pull request overview

This pull request improves PSGallery publishing diagnostics with package-aware messages for authentication, permission, and conflict errors.

Changes:

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

File summaries

File	Summary
src/code/RepositorySettings.cs	Makes the PSGallery URI available to publishing logic.
src/code/PublishHelper.cs	Adds package-aware error handling. Findings include incorrect 401 messaging, missing message assertions for 401/403/409 responses, trailing-slash URI normalization, and duplicate terminal periods in two diagnostics.

Review details

Suppressed comments (3)

src/code/PublishHelper.cs:811

• This renders modified.)., with two terminal periods, so the new duplicate-package diagnostic is malformed. Keep only one terminal period in this reason phrase.

                                $"(A package with id '{packageName}' and version '{packageVersion}' already exists and cannot be modified.).",

src/code/PublishHelper.cs:787

• This renders package.)., with two terminal periods, so the diagnostic is malformed and does not preserve a clean PSGallery reason phrase. Keep the period either inside the parenthesized reason or after the closing parenthesis, not both.

                            new ArgumentException($"Could not publish to repository '{repoName}'. Response status code does not indicate success: 403 (The specified API key is invalid, has expired, or does not have permission to access the specified package.)."),

src/code/PublishHelper.cs:709

• This trims the URI only for isPSGallery; publishLocation was already computed from the untrimmed repoUri at line 670. A PSGallery URI ending in / (which can be registered with -ApiVersion V2) is therefore still published to /api/v2/ instead of /api/v2/package, while this branch treats it as PSGallery. Normalize before constructing publishLocation and reuse that value for both operations.

                string normalizedRepoUri = repoUri.TrimEnd('/');
                bool isPSGallery = normalizedRepoUri.Equals(RepositorySettings.PSGalleryRepoUri, StringComparison.OrdinalIgnoreCase);

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

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