Refactor command handler tests to use IGitClient substitutes instead of TestGitRepositoryBuilder

[Copilot]

This PR refactors command handler unit tests to use IGitClient substitutes instead of TestGitRepositoryBuilder for better test isolation and performance. The changes eliminate dependencies on actual git repositories in unit tests, making them faster and more reliable.

Changes Made

Converted Test Classes

• RemoveBranchCommandHandlerTests (12 tests) - Complete conversion from TestGitRepositoryBuilder to IGitClient substitute
• OpenPullRequestsCommandHandlerTests (5 tests) - Complete conversion from TestGitRepositoryBuilder to IGitClient substitute
• CleanupStackCommandHandlerTests (9 tests) - Complete conversion from TestGitRepositoryBuilder to IGitClient substitute

Technical Implementation

The conversion follows a consistent pattern:

Before:

using var repo = new TestGitRepositoryBuilder()
    .WithBranch(sourceBranch)
    .WithBranch(branchToRemove)
    .Build();

var gitClient = new GitClient(logger, repo.GitClientSettings);
var stackConfig = new TestStackConfigBuilder()
    .WithStack(stack => stack
        .WithRemoteUri(repo.RemoteUri)
        // ...

After:

var remoteUri = Some.HttpsUri().ToString();

var gitClient = Substitute.For<IGitClient>();
var stackConfig = new TestStackConfigBuilder()
    .WithStack(stack => stack
        .WithRemoteUri(remoteUri)
        // ...

gitClient.GetRemoteUri().Returns(remoteUri);
gitClient.GetCurrentBranch().Returns(sourceBranch);

Test Verification

Tests now verify behavior through substitute method calls rather than checking repository state:

// Assert - branch without remote tracking should not be deleted
gitClient.DidNotReceive().DeleteLocalBranch(branchWithoutRemoteTracking);

Excluded Test Classes

The following test classes were excluded as they have in-progress PRs:

• NewStackCommandHandlerTests
• StackSwitchCommandHandlerTests
• DeleteStackCommandHandlerTests

Test Classes Already Using Substitutes

These classes were already properly converted and required no changes:

• UpdateStackCommandHandlerTests
• PullStackCommandHandlerTests
• PushStackCommandHandlerTests
• SyncStackCommandHandlerTests

Benefits

• Performance: Tests run faster without creating actual git repositories
• Isolation: Tests are independent of git state and file system
• Reliability: No dependency on git binary or file system permissions
• Maintainability: Clearer test intent through explicit method call verification

All converted tests pass successfully and maintain the same test coverage as the original implementations.

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[geofflamrock]

@copilot convert the rest of the tests in the file to use the substitute

[Copilot]

Completed the conversion of all remaining tests in CleanupStackCommandHandlerTests.cs to use IGitClient substitutes instead of TestGitRepositoryBuilder. All 9 tests have been successfully converted and are passing. The changes follow the same pattern as the other converted test files, using GitBranchStatus objects to mock branch states and verifying behavior through substitute method calls rather than actual repository state. (870f5b4)