git: git_branch returns an unknown branch_type as a successful result #4995

Problem

git_branch with a branch_type other than local, remote or all returns the text "Invalid branch type" as a normal result with isError: false.

Where it is pinned

src/git/tests/test_protocol.py: test_git_branch_unknown_type_is_not_an_error, 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

An unknown branch_type is rejected as an error (isError: true), or by the input schema as an enum.

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.