test: respect Windows argument encoding in packed-tag regressions

The Windows Python 3.15 free-threaded CI job enumerated the non-UTF-8
packed tags correctly, then failed both object-format variants when the
new regression tried to resolve their surrogate-escaped names through
the CLI. Git 2.56.0.windows.2 returned exit 128 from `git rev-parse
--verify --end-of-options` with `fatal: Needed a single revision`.
Git's Windows `wmain()` converts UTF-16 arguments to UTF-8, so those
arguments cannot carry the original invalid UTF-8 reference bytes.

Separate packed-tag enumeration from reference resolution. Keep the
reported enumeration regression on both backends and every platform;
skip only the Windows CLI resolution variant. Gix resolution still runs
on every platform with `Git.execute()` forbidden, using the supported
byte-capable native APIs. Document the demonstrated CLI limitation and
the coverage boundary in `doc/gix-backend.md`.

Validation: all 44 reference tests and 24 subtests pass locally on each
backend; Ruff check/format and `git diff --check` pass. Windows execution
remains for CI.

Author: codex

doc/gix-backend.md

@@ -393,10 +393,17 @@ Reference names and revisions are encoded with `surrogateescape` before calling
 GixPython's byte-capable `Target.Symbolic`, `Repository.try_find_reference`,
 `Repository.rev_parse_single`, `Repository.rev_parse` and `RefEdit` APIs.
 This preserves non-UTF-8 packed tag names without interpreting reference storage
-in Python. `test_non_utf8_tag_names_preserve_bytes_and_resolve` covers lightweight
-and annotated tags in SHA-1 and SHA-256 repositories and forbids CLI fallback
-during native resolution. This corrects adapter input conversion; GixPython 0.3.0
-already accepts the original bytes through those APIs.
+in Python. `test_non_utf8_tag_names_preserve_bytes` covers lightweight and annotated
+tag enumeration in SHA-1 and SHA-256 repositories on both backends and all platforms.
+`test_non_utf8_tag_names_resolve` forbids CLI fallback during native resolution.
+Its CLI variant requires POSIX: Windows command-line arguments cannot preserve
+the original non-UTF-8 bytes. Windows CI with Git 2.56.0.windows.2 and CPython
+3.15.1 free-threaded enumerates both packed names, but resolving the
+surrogate-escaped `annotated-\udce3` name through `git rev-parse --verify` exits 128
+with `fatal: Needed a single revision`. Enumeration remains tested on Windows
+CLI; Gix resolution remains tested on Windows through the byte-capable APIs.
+This corrects adapter input conversion; GixPython 0.3.0 already accepts the
+original bytes through those APIs.
 
 Repository-backed native operations fall back for reftable (GIX-1) and repositories
 with `extensions.compatObjectFormat` (GIX-19). Process-returning calls, timeouts,

test/test_refs.py

@@ -4,6 +4,7 @@
 # 3-Clause BSD License: https://opensource.org/license/bsd-3-clause/
 
 import contextlib
+import os
 import os.path as osp
 import unittest
 from itertools import chain
@@ -37,11 +38,23 @@
 
 
 @pytest.mark.parametrize("object_format", ["sha1", "sha256"])
-def test_non_utf8_tag_names_preserve_bytes_and_resolve(object_format):
+def test_non_utf8_tag_names_preserve_bytes(object_format):
     with fixture_repo("legacy-refs.sh", object_format) as repo:
         tags = repo.tags
         assert len(tags) == 2
         assert {safe_encode(tag.path) for tag in tags} == {b"refs/tags/release-\xf5", b"refs/tags/annotated-\xe3"}
+        if _backend.name == "gix":
+            with patch.object(_backend, "gix", None):
+                assert {tag.path for tag in repo.tags} == {tag.path for tag in tags}
+
+
+@pytest.mark.skipif(
+    os.name == "nt" and _backend.name == "cli", reason="Windows command-line arguments cannot preserve non-UTF-8 bytes"
+)
+@pytest.mark.parametrize("object_format", ["sha1", "sha256"])
+def test_non_utf8_tag_names_resolve(object_format):
+    with fixture_repo("legacy-refs.sh", object_format) as repo:
+        tags = repo.tags
         native_guard = (
             patch.object(Git, "execute", side_effect=AssertionError("unexpected reference CLI call"))
             if _backend.name == "gix"
@@ -52,9 +65,6 @@ def test_non_utf8_tag_names_preserve_bytes_and_resolve(object_format):
                 assert tag.commit == repo.head.commit
                 assert repo.rev_parse(tag.path).hexsha == tag.object.hexsha
                 assert repo.commit(tag.path) == repo.head.commit
-        if _backend.name == "gix":
-            with patch.object(_backend, "gix", None):
-                assert {tag.path for tag in repo.tags} == {tag.path for tag in tags}
 
 
 class TestRefs(TestBase):