Create_Release_Git_Tag: CI-aware, idempotent tagging, and dry-run support

[Copilot]

This PR enhances the Create_Release_Git_Tag build task with three key improvements:

1. CI-aware commit resolution

• Adds a new BuildCommit parameter that automatically resolves the commit SHA from CI environment variables
• Supports GitHub Actions (GITHUB_SHA) and Azure Pipelines (BUILD_SOURCEVERSION)
• Falls back to git rev-parse HEAD if no CI variables are available
• Ensures release tags point to the exact commit that was built/tested, avoiding issues where the tag points to a different commit than what was actually deployed

2. Idempotent tagging behavior

• Checks for existing remote tags before attempting to create new ones
• Skips tag creation if the tag already exists remotely, preventing conflicts in overlapping CI runs
• Validates that target commits exist locally and fetches them if necessary

3. Dry-run support

• Adds a new DryRun parameter (defaults to false) for testing tag operations without making changes
• When enabled, skips the actual push and verification commands
• Outputs clear messages showing what would have been done
• Returns the commit SHA that would have been tagged

Enhanced logging

• Provides detailed debug output showing the resolved commit SHA and its source
• Logs all key operations including fetching, tag creation, and verification
• Makes troubleshooting easier in CI environments

These changes maintain backward compatibility while providing more reliable and observable tag creation in CI/CD pipelines.

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This change is 

[johlju]

@copilot update changelog with a short entry

[Copilot]

@copilot update changelog with a short entry

Added changelog entry under "Unreleased > Added" section describing the Create_Release_Git_Tag task enhancement. Commit: d9ca6d4

[johlju]

I just realized that it is not this task that sets the preview tag for GitHub, it is the task Publish_release_to_GitHub and this line in particular: https://github.com/gaelcolas/Sampler.GitHubTasks/blob/1f288c3bc35ab124ff9f8a8f405111c10761744e/.build/tasks/New-Release.GitHub.build.ps1#L125

[johlju]

I keep this open for a while if there is any demand we can merge, but need someone to verify functionality in an actual use-case.