Move UpdateStack to separate interface to improve testing

[geofflamrock]

Moves StackHelpers.UpdateStack into StackActions to improve the ability to test commands without needing to have a full Git repo.

Part of a series of PRs:

• Cleanup unused methods in GitClient #323
• Move PullChanges to separate interface to improve testing #326
  • Move PushChanges to separate interface to improve testing #327
    • Move UpdateStack to separate interface to improve testing #328
      • Cache GitHub pull requests to improve performance #329
• Refactor delete stack tests to not need a full Git repo #330
• Refactor new stack tests to not use full Git repo #332
• Refactor stack switch tests to not use full Git repo #333
• Refactor add branch tests to not use full Git repo #341
• Refactor new branch tests to not use full Git repo #342
• Refactor stack status tests to not use a full Git repo #346
• Change more push stack tests to not use a full Git repo #347
• Change create pull requests tests to not use full Git repo #349

[Copilot]

Pull Request Overview

This PR refactors the stack update functionality by moving StackHelpers.UpdateStack into a separate StackActions interface to improve testability. Commands now use the IStackActions interface instead of directly calling static methods, enabling easier unit testing without requiring full Git repositories.

• Moved UpdateStack functionality from StackHelpers to StackActions class
• Updated multiple command handlers to use the new IStackActions interface
• Converted integration tests to unit tests using mocked dependencies

Reviewed Changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
UpdateStackCommand.cs	Modified to use new StackActions interface with additional dependencies
SyncStackCommand.cs	Updated constructor to use new StackActions interface
PushStackCommand.cs	Updated StackActions constructor call with new parameters
PullStackCommand.cs	Updated StackActions constructor call with new parameters
StackHelpers.cs	Added GetUpdateStrategy method to extract strategy determination logic
StackActions.cs	Added UpdateStack method and supporting merge/rebase implementation
UpdateStackCommandHandlerTests.cs	Converted integration tests to unit tests using mocks
SyncStackCommandHandlerTests.cs	Converted integration tests to unit tests using mocks
StackHelpersTests.cs	Removed tests that were moved to StackActionsTests
StackActionsTests.cs	Added comprehensive unit tests for new UpdateStack functionality

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[Copilot]

The UpdateBranchLineUsingMerge method is marked as public but appears to be an internal implementation detail. Consider making it private to reduce the public API surface.

Suggested change

	public void UpdateBranchLineUsingMerge(
	private void UpdateBranchLineUsingMerge(