Windows cross generator actions now invoke their source with the downloaded Linux Python runtime, with declared runtime and import inputs. Consolidate python_genrule with the shared template renderer and include IDL wrapper runfiles; preserve the reviewed native Python environment.

Extracted from #63670, following merged #64688. Native Windows remains the default.

Validation: six Bazel analysis/runtime tests passed with the Linux x86_64 runtime enabled; real error-code and Python fixture generation builds passed; repository formatting passed. Windows cross execution and the macOS IDL wrapper test remain for CI.