fix(uv): move venv aside instead of gutting it in place on Windows rebuild

hermes update can brick a Windows install. When 'hermes update --force' runs
past the concurrent-process guard, rebuild_venv runs while the venv is still in
use: shutil.rmtree(ignore_errors=True) deletes site-packages + certifi's cert
bundle but can't remove the locked python.exe, leaving a half-gutted venv that
uv venv then refuses to overwrite. Every later HTTPS call dies with
FileNotFoundError for the missing cacert and there is no recovery.

--clear alone (the c136eb4de retry path) does not fix the real lock case: when
the locked interpreter is *inside* the venv being rebuilt, neither rmtree nor
uv venv --clear can delete it. os.replace of the parent directory *is* allowed
on Windows (a running .exe is tracked by handle, not path), so we move the old
venv aside atomically to <venv>.old, rebuild with --clear in its place, and the
still-running gateway/desktop keep using the moved-aside copy until they
restart. If the venv genuinely can't be moved, we abort cleanly and leave it
fully intact; if the rebuild fails, we restore the moved-aside copy.

Folds in the call-site guards from #38511 (@f3rs3n):
- rebuild_venv() returns False (and restores the backup) if uv exits 0 without
  producing an interpreter.
- both hermes update venv-rebuild call sites abort with RuntimeError instead of
  continuing into dependency install when rebuild_venv() returns False.

Also gitignore /venv.old/ so the update autostash (git stash --include-untracked)
doesn't sweep the moved-aside venv on every run.

Root-cause fix for #37881. Supersedes the --clear-only retry from c136eb4de.

Co-authored-by: f3rs3n <32328813+f3rs3n@users.noreply.github.com>

.gitignore

@@ -1,5 +1,6 @@
 .DS_Store
 /venv/
+/venv.old/
 /_pycache/
 *.pyc*
 __pycache__/