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/tests/protocol-mirror.e2e.ts

@@ -53,8 +53,9 @@ describe.skipIf(!python3Available)('protocol.py mirrors protocol.ts at runtime',
 })
 
 it('names the py/ directory that ships with the package', () => {
-  // The package.json `files` list ships `py/**/*.py`; the mirror test resolves
-  // the marker source relative to the built package, so the directory must exist
-  // beside the tests even when python3 is absent from the runner.
+  // Resolves py/ relative to this test file; the same directory ships in the
+  // package.json `files` whitelist (`py/**/*.py`). The tests/ directory itself
+  // is not published — this asserts the source-tree layout the mirror test
+  // depends on, so it holds even when python3 is absent from the runner.
   expect(existsSync(pyDir)).toBe(true)
 })