refactor(tools): pin the two language tables to one union, and name python at the seam

`SDK_RENDERERS` and `RUN_CODE_FLAVORS` had to stay in step by review alone:
the `Object.hasOwn` guards catch drift only once a runtime reporting the
half-added language exists, which is the one case that cannot arise. Both
tables are now `satisfies`-checked against a shared `CodeSdkLanguage` union,
so a missing or extra entry fails `typecheck`. The declared `Record<string, …>`
type stays, since `CodeRuntime.language` is an unconstrained `string`.

The code-runtime seam's own README row and `CodeRuntime.language` JSDoc still
named `'typescript'` as the sole well-known value; both now name `'python'`
too and say only `'typescript'` has a published backend.

packages/code-runtime/code-runtime/README.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write packages/code-runtime/code-runtime/README.md
-README.md: c7a2d519e47d160f5ab123bfc887e7e9f24ec602
-README.zh.md: 22d0b120d7cea50b578a184b3e40d77707ebc489
+README.md: ec962d7def4bc751151d417fd5a7026038814f33
+README.zh.md: a94ea0feed18f2c7dd99816f072645eebe197e97