Add Regenerate feature to Update workflow using shared helper

[samvaity]

• Extended ITspClientHelper with UpdateGenerationAsync
• Implemented update command in TspClientHelper invokes [tsp-client update --tspPath ... --output-dir ...]
• Injected ITspClientHelper into TspClientUpdateTool and replaced placeholder regenerate step with real call to generate new code at provided newGen path location if provided.

[Copilot]

Pull Request Overview

This PR adds regeneration capability to the TypeSpec client update workflow by integrating with the tsp-client CLI tool. It replaces the placeholder regeneration step with actual code generation functionality.

Key changes:

• Extended the ITspClientHelper interface with UpdateGenerationAsync method to invoke tsp-client update commands
• Refactored TspClientUpdateTool to accept commit SHA instead of spec path and implemented actual regeneration logic
• Added logic to update tsp-location.yaml files with new commit SHA and handle output directory resolution

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
TspClientUpdateTool.cs	Refactored to use commit SHA, added regeneration logic using ITspClientHelper, and improved directory handling
TspClientUpdateResponse.cs	Added Failed enum value to UpdateStage for error handling
TspClientHelper.cs	Implemented UpdateGenerationAsync method to execute tsp-client update commands
ITspClientHelper.cs	Extended interface with UpdateGenerationAsync method and updated documentation
TspClientUpdateToolTests.cs	Updated tests to use mock ITspClientHelper and commit SHA values instead of spec paths