chore(python): audit tests

Split the three oversized test files into support modules and topic
files, evaluate the rmdir dir_fd capability once at import, and drop
unused imports.

Claude-Session: https://claude.ai/code/session_01Agoc9bRVE9KL7mmWtey9Vq

tests/transaction_support.py

@@ -0,0 +1,42 @@
+from __future__ import annotations
+
+import os
+from pathlib import Path
+
+
+from elydora.plugins import _transaction
+
+
+def write_source(path: Path, source: str) -> None:
+    path.write_bytes(source.encode("utf-8"))
+
+
+def make_change(
+    path: Path,
+    original: str | None,
+    next_source: str | None,
+    mode: int = 0o600,
+    enforce_mode: bool = False,
+) -> _transaction.FileChange:
+    change = _transaction.source_change(
+        str(path),
+        path.name,
+        original,
+        next_source,
+        mode,
+        enforce_mode=enforce_mode,
+    )
+    assert change is not None
+    return change
+
+
+def transaction_subprocess_environment(home: Path) -> dict[str, str]:
+    environment = os.environ.copy()
+    environment["HOME"] = str(home)
+    environment["USERPROFILE"] = str(home)
+    return environment
+
+
+def swap_directory(parent: Path, backup: Path, redirected: Path) -> None:
+    parent.rename(backup)
+    redirected.rename(parent)