…aising AssertionError (Unstructured-IO#4597)

## Summary

`partition_email()` processes attachments by default. When a message has
a
`message/rfc822` part, the whole call fails with a bare `AssertionError`
and
the body elements are lost too. Common inputs have such a part:

- an email forwarded "as attachment" (Outlook, Thunderbird),
- a delivery-failure bounce (`multipart/report` with the returned
message),
- a `multipart/digest`, where every part is `message/rfc822`.

The repository's own fixture
`example-docs/eml/mime-multipart-digest.eml`
fails this way, through `partition()` too.

Cause: for a `message/rfc822` part, `get_content()` returns the enclosed
`EmailMessage`, not `str` or `bytes`, so
`_AttachmentPartitioner._file_bytes`
fails its `assert isinstance(content, bytes)`. The assert sits outside
the
`try`/`except` that skips attachments it cannot partition.

Fix, in `unstructured/partition/email.py`:

- `_file_bytes` serializes an enclosed message with `as_bytes()`.
- For a `message/rfc822` part, `content_type="message/rfc822"` is added
to the
kwargs of the nested `partition()` call. Without it the bytes go through
libmagic, which reports a message as `text/plain` when its first header
is
e.g. `MIME-Version`, `Message-ID` or `To` and not
`From`/`Received`/`Date`.
The attachment's raw headers then come out as `Title` elements. Python's
`EmailMessage` writes such messages: the forwarded message in the new
test
starts with `Content-Type`, which libmagic does not recognize either.
The
hint is merged into the kwargs only for these parts, so a
`content_type=`
  kwarg passed to `partition_email()` directly still reaches the other
  attachments as before instead of colliding with an explicit keyword.

Related:

- Unstructured-IO#4424 is the sibling fix in the same `_file_bytes` function (the
`KeyError`
on a `multipart/*` attachment). Its comment already lists
`message/rfc822`
among the types `get_content()` handles, but that path still hit the
assert.
- Unstructured-IO#3604 (closed in October 2024 with no fix linked) reported
`process_attachments=True` failing on a nested `.eml`, with a
`TypeError` in
the implementation of that time. A maintainer asked there for an email
that
reproduces it so a test could guard the behavior. The tests here use two
such emails: the existing digest fixture and a forwarded message built
in
  the test.

## Validation

Environment: `main` at c74e6c4, Python 3.12,
`uv sync --locked --group test` (no extras; libmagic installed).

New tests in `test_unstructured/partition/test_email.py` (attachments
section):

- `test_partition_email_partitions_the_messages_of_a_multipart_digest`:
uses
  the existing `mime-multipart-digest.eml` fixture. It checks the three
  messages' text, their `subject` metadata and `attached_to_filename`.
-
`test_partition_email_partitions_a_message_forwarded_as_an_attachment`:
an
  `EmailMessage` forwarded with `add_attachment()` whose first header is
  `Content-Type`. It checks the body, the forwarded text, `subject` and
  `sent_from`.

Results:

- New tests without the fix:
`pytest test_unstructured/partition/test_email.py -k "multipart_digest
or forwarded_as_an_attachment"`
gives 2 failed, both `AssertionError` at
`unstructured/partition/email.py:485`
(`assert isinstance(content, bytes)`). The whole file without the fix
gives
  2 failed, 77 passed, 1 skipped.
- New tests with the fix: 2 passed.
- With only the `as_bytes()` change and no `content_type` hint, the
digest
  test passes and the forwarded-message test fails with 6 extra `Title`
elements (the forwarded message's raw headers). That is why the hint is
  part of the fix.
- `pytest test_unstructured/partition/test_email.py`: 79 passed, 1
skipped.
  The skip is the WEBP test that needs `unstructured_inference`.
- `pytest test_unstructured/file_utils/test_filetype.py
test_unstructured/partition/test_msg.py`:
  362 passed, 1 failed. The failure is
`test_partition_msg_can_process_attachments`, which fails the same way
on
  `main` because the `pptx` extra is not installed. `msg.py` has its own
  attachment partitioner and is not changed.
- Manual check of a bounce (`multipart/report` with `text/plain`,
`message/delivery-status` and a `message/rfc822` part whose first header
is
`MIME-Version`): returns the bounce text, the delivery-status lines and
the
returned message's body with `subject == "Lunch"`. Before the fix it
raised
`AssertionError`. A forward of a forward is also partitioned at every
level.
- `make check` (ruff 0.15.10 from the `lint` group): `All checks
passed!`,
  `355 files already formatted`.
- `bash scripts/test-check-new-release-version.sh`: `Passed 27 version
guard tests.`
- `VERSION_BASE_REF=upstream/main bash
scripts/check-new-release-version.sh`:
  `PASS: unchanged development version 0.27.35-dev (no tag lookup).`
- No `CHANGELOG.md` or `unstructured/__version__.py` changes.

## Release note

`partition_email()`, and `partition()` on an `.eml` file, now partition
an
attached email (a message forwarded as an attachment, the returned
message in
a bounce, each message of a `multipart/digest`) like any other
attachment.
Before, they raised `AssertionError`. Each element from the attached
message
carries that message's own `subject`, `sent_from` and other email
metadata.

## Checklist

- [x] The PR title is a user-facing release-note entry with a
conventional commit prefix.
- [ ] Apply a category label (`enhancement`, `bug`, or an appropriate
existing label), or ask a maintainer to apply it. A maintainer needs to
apply `bug`.
- [ ] For breaking changes, use `!` in the title and explain impact and
migration above. (Not a breaking change.)

---------

Co-authored-by: sinnercode228 <[email protected]>