docs(code-runtime-python): register the truncation exception to open merging; clean a case comment

The review's warning: the READMEs (outer and inner wire sections, en + zh) and
the fd-3 protocol note still claimed the next log frame always merges into an
open entry, while truncateLogs commits the already-billed prefix as its own
entry before the marker. The one exception (truncation) is now stated in both
READMEs and the owning note, paired and re-recorded. The prefix-commit case's
parenthetical describing the pre-fix implementation is removed per the
comment-does-not-record-review-history rule.

packages/code-runtime/code-runtime-python/tests/runtime.spec.ts

@@ -1901,8 +1901,7 @@ describe('PythonCodeRuntime — programs and bindings', () => {
   it('commits a flushed open prefix before the truncation marker', async () => {
     // A flushed unterminated line is billed and committed; when a later
     // over-budget write truncates, the committed prefix must appear BEFORE the
-    // marker — the ledger charged for it, so it cannot vanish. (The bug: all
-    // truncation arms pushed only the marker, dropping the held prefix.)
+    // marker — the ledger charged for it, so it cannot vanish.
     const { runtime } = await setup({ maxLogBytes: 64 })
     const result = await runtime.run({
       program: [