fix(code-runtime-python): close coverage gap and tighten the wire mirror

- Cover the log-frame `truncated` rebuild branch: assert a literal-true flag
  rides along and any other value (1, string, false) is dropped, closing the
  protocol.ts branch the coverage gate flagged.
- Correct encodeJsonPlain's JSDoc: it matches compact JSON.stringify EXCEPT on
  a beyond-safe-range integral double, where it emits the exact BigInt digits
  (`...846976`) rather than the rounded `...847000` — the divergence the
  "emits exact digits" test pins.
- Declare py/protocol.py's `global`-bearing frames (Namespace, CallMessage)
  with functional TypedDict syntax so they carry the real wire key instead of
  a `global_` attribute the wire never sends, and split optional-field messages
  (Namespace/LogMessage/DoneMessage) into a required base plus a total=False
  subclass so `type` and other required fields cannot be dropped. Widen
  HostToChild to include the boot and run frames the host sends before replies.
- Reword the mirror e2e's py/ directory assertion to describe the source-tree
  layout it actually checks.

packages/code-runtime/code-runtime-python/src/protocol.ts

@@ -124,7 +124,11 @@ export function logTruncationMarker(maxBytes: number): string {
  * (the worker backend's wire is equally stack-safe). Callers must pass a value
  * produced by `JSON.parse` (or equally JSON-plain): only `null`, finite
  * numbers, booleans, strings, dense arrays, and plain objects — this encoder
- * validates nothing. Output is byte-identical to compact `JSON.stringify`.
+ * validates nothing. Output matches compact `JSON.stringify` byte for byte
+ * EXCEPT on an integral double beyond the safe range, where {@link scalarJson}
+ * emits the exact integer's BigInt digits rather than `JSON.stringify`'s rounded
+ * spelling (`1152921504606846976`, not `...847000`) so the seam's lossless-JSON
+ * promise holds across the wire.
  * @param value - a JSON-plain value (e.g. straight from `JSON.parse`).
  * @returns the compact JSON encoding.
  */