tests: make bad-tag recovery test portable across Git versions #350

Summary

git_ops::tests::test_ensure_remote_bad_tag_recovery fails consistently with
Git 2.25.1 because git fetch --prune --no-tags succeeds without inspecting or
removing the deliberately corrupt local tag.

The recovery logic is entered only when fetch fails with stderr containing
fatal: bad object refs/tags/. On this Git version that diagnostic is not
produced, so ensure_remote() returns successfully while
.git/refs/tags/bad-tag remains and the final assertion fails.

Reproduction

On current main at 8ac8e51fd79c8560404efe62d0c9ce38defce5af:

cargo test --release --lib \
  git_ops::tests::test_ensure_remote_bad_tag_recovery -- --exact

Result:

test git_ops::tests::test_ensure_remote_bad_tag_recovery ... FAILED

Bad tag should have been deleted by recovery logic

The failure also causes make check-pr to fail locally.

Environment

• Rust 1.90.0
• Git 2.25.1
• WSL Ubuntu

Expected behavior

The test should deterministically exercise the recovery path across supported
Git versions, or the repository should document and enforce a minimum Git
version that produces the expected fetch failure.

This was discovered while validating an unrelated NNTP whitespace fix. That
change does not modify src/git_ops.rs, and the failure reproduces when this
test is run alone.