Add GitHub Actions workflow for updating static web assets baselines

[Copilot]

This PR adds a GitHub Actions workflow that automates the process of updating static web assets baselines for pull requests, eliminating the need for manual intervention by developers.

Features

The workflow (update-static-web-assets-baselines.yml) provides:

• Manual dispatch trigger with PR number input for targeted baseline updates
• Automated PR branch checkout using GitHub CLI
• Full project build via ./build.sh -c Release
• Proper environment setup with DOTNET_ROOT configuration
• Baseline update execution using src/RazorSdk/update-test-baselines.sh
• Smart change detection for JSON files in test directories
• Automatic commit and push with standardized commit messages
• PR status feedback through automated comments

Usage

Developers can trigger the workflow using GitHub CLI:

# Update baselines for PR #1234
gh workflow run update-static-web-assets-baselines.yml -f pr_number=1234

# Monitor workflow progress
gh run watch

Workflow Behavior

The workflow provides clear feedback through PR comments:

• "Baselines updated." - When changes are successfully committed and pushed
• "No baselines were updated." - When baselines are already up-to-date
• "Update baselines failed: [reason]" - When errors occur during the process

Error Handling

The workflow includes comprehensive error handling for:

• Build script failures
• Baseline update script failures
• Git commit/push failures

Each failure scenario provides specific error messages to help developers diagnose issues.

Implementation Details

The workflow follows established patterns in the repository:

• Uses actions/checkout@v4 for repository access
• Leverages GitHub CLI for PR operations
• Implements proper permissions for content and PR management
• Uses continue-on-error for graceful failure handling
• Follows the existing commit message format: [Infrastructure] Update baselines YYYY-MM-DD

This automation significantly streamlines the baseline update process, reducing manual overhead and potential errors while maintaining consistency across the development workflow.

Fixes #49729.

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