fix: populate sha256 in resource files and fix Windows PythonConfiguration

- Add sha256 field to Windows PythonConfiguration (PyPy/GraalPy have
  direct download URLs on Windows too)
- Pass sha256 to install_pypy() and install_graalpy() in windows.py
- Fix update_pythons.py: handle empty sha256 from CPython API (older
  versions) by streaming download to compute it; fix condition to
  check 'not sha256' rather than 'not in dict'
- Fix update_virtualenv.py: compute sha256 even when version unchanged
  but sha256 is empty (first-time population)
- Fix update_python_build_standalone.py: resolve file path relative to
  the script itself (not the installed package) so writes go to source
  checkout, not the uv cache
- Populate actual sha256 values by running all three update scripts

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Assisted-by: copilot-cli:claude-sonnet-4.6

bin/update_pythons.py

@@ -535,9 +535,11 @@ class AllVersions:
 
         # Fill in sha256 for URL-based configs if not already provided by the
         # update_version_* method (e.g. PyPy, BeeWare iOS, Maven have no sidecar).
+        # Also fills in sha256 when the CPython API doesn't return a sha256_sum
+        # (e.g. for older releases).
         # Widen the type to allow arbitrary key access on the underlying dict.
         config_update_dict: dict[str, str] = config_update  # type: ignore[assignment]
-        if "url" in config_update_dict and "sha256" not in config_update_dict:
+        if "url" in config_update_dict and not config_update_dict.get("sha256"):
             url = config_update_dict["url"]
             existing_sha256 = config.get("sha256", "")
             if url == config.get("url") and existing_sha256: