git: git_create_branch reports "Created branch" for a branch that already exists at the base #4996

Problem

When a branch with the requested name already exists and already points at the base commit, git_create_branch reports "Created branch …" although nothing was created.

Where it is pinned

src/git/tests/test_protocol.py: test_git_create_branch_existing_name_at_same_commit_reports_success, added in #4974 (Wave 1 of #4857). Each of these tests carries a KNOWN BUG marker: it asserts the current, wrong behavior, so the fix has to change it.

Expected

Creating a branch whose name already exists is reported as an error (or at least not as a creation).

Done when

• The behavior above is fixed, in the legacy-era (2025-11-25) server on SDK 1.x.
• The pinning tests assert the correct behavior, and their KNOWN BUG markers are gone.
• The per-file 90% coverage gate still passes.