Add streaming document download (stream_document_content)

[baraline]

Gap

download_document_content (_async/clients/api/management/_document.py:215) returns response.content
after a non-streaming dispatch through self._session.request(...). httpx's send defaults to
stream=False and then awaits response.aread(), so the whole body is materialised in memory. No public
surface exposes streaming or the session, and no workaround is documented.

Proposed API

async def stream_document_content(
    self, document_id: GlpiId, *, chunk_size: int = 65536
) -> AsyncIterator[bytes]: ...

Codegen cost: one token

Verified by running the real unasync over a streaming probe:

async with     -> with          FREE (the bare `async` token is stripped)
async for      -> for           FREE
AsyncIterator  -> Iterator      FREE (unasync's built-in table)
aiter_bytes    -> aiter_bytes   NOT REWRITTEN   <-- the only rule needed

Three coordinated edits:

1. "aiter_bytes": "iter_bytes" in TOKEN_REPLACEMENTS (unasync_build.py:88-99)
2. "aiter_bytes" in _INTENTIONAL_RENAMES (testing/tests/test_unasync_codegen.py:44-55) in the same
   commit -- exactly the pattern aclose/aread already follow. Without it the guard reports
   ('new.py', 5, 'aiter_bytes').
3. A test-harness change that is easy to miss: testing/tests/test_method_invocation.py:169 installs
   its stub only at client._session.request, and httpx.Client.stream does not route through
   Client.request. A public stream_document_content would fail
   test_every_public_method_reaches_the_transport on both surfaces and attempt a real socket connect to
   https://glpi.example.test. _install_stub needs a .stream stub -- hand-written for each surface
   (a @contextmanager for sync, an @asynccontextmanager for async), since that file is not
   unasync-generated.

Two further gotchas

• ensure_response_status reads response.text (_http.py:281,284), which raises ResponseNotRead on an
  unread stream. Call aread()/read() first -- aread is already a substitution key.
• Do not decorate the generator with @retry: per
  docs/superpowers/specs/2026-07-15-real-async-io-design.md:345-346, tenacity silently degrades to the
  sync path on an async generator.

Scope note

aiter_text / aiter_lines / aiter_raw are equally un-rewritten. Only aiter_bytes is needed for this
API; anyone generalising to text or line streaming needs the rest.