Remove obsolete Git test seams - #14355

N/A. This is the final code layer above draft PR #14351.

Description

Layers 1-4 moved Git client tests onto real local repositories and focused command assertions, but the package still carried two subprocess mock protocols and a private Client.commandContext production field used only by tests.

This removes that field and its commandCtx alias, deletes the rich JSON/environment command-mocking protocol, and replaces the remaining uses with observable real-Git state, the existing CommandModifier seam, or a narrow malformed-output parser. The one surviving subprocess helper now accepts explicit arguments instead of test-only environment variables.

How did you test this change?

Not tested manually because this is a behavior-preserving cleanup of internal test infrastructure with no user-visible command change.

Key points

• Removed Client.commandContext, commandCtx, createCommandContext, createMockedCommandContext, TestHelperProcess, TestCommandMocking, commandResult, mockedCommands, and all GH_*HELPER_PROCESS* variables from git tests. createCommitsCommandContext and TestCommitsHelperProcess already had zero remaining definitions or callers at the layer-5 baseline.
• Retained one focused TestGitCommandHelperProcess plus createCommand for Command.Output stdout/stderr/exit behavior and recordCommandArgs for operation-to-auth wiring. These remain necessary to protect exact credential-helper clearing/scoping, pull --ff-only, push --set-upstream, and clone host scoping without executing a fake Git path.
• Moved RemoteURL and IsIgnored option-separator checks to observable real-Git state. Moved malformed push-revision output to a narrow parser seam. Real missing repository directories now prove arbitrary Git error propagation.
• Preserved command modifiers, missing executable behavior, local-only Git transport coverage, literal Given/When/Then scenarios, and the layer-4 test shape.
• Baseline git package metrics were 9.058s normal, 19.174s race, and 86.1% coverage. Final uncontended metrics were 8.074s normal, 14.688s race, and 86.4% coverage.
• Net diff: 78 additions, 185 deletions, 107 fewer lines across 4 files.
• Deliberately deferred moving git.IsolateConfig out of git/test.go: no existing internal test-support package fits, and introducing one would only relocate the production testing dependency while touching three consumer packages and exporting a test-only internal API.

Notes for reviewers

Start with Client.Command in git/client.go, then review the authenticated operation tests and real-Git option-boundary scenarios in git/client_test.go. This PR targets williammartin-improve-git-tests, the branch for draft PR #14351. Stack metadata is intentionally unchanged.

Authorship and follow-up

Who wrote this:

• A human wrote it.
• An agent wrote it under close human direction.
• An agent wrote it independently, and no human has guided the implementation beyond the initial prompt.

Who answers review comments:

• @williammartin will read and reply directly. Name the account.
• An agent will draft replies and @username will read them before they are posted.
• Nobody has explicitly committed to replying.