Release 2.0.16: backport channelData optional-subfield fix (#564) - #566

Fixes #563

The Teams service can send nested `channelData` objects as empty objects
( `{}` ). Because `AppInfo.id` was required, Pydantic raised a
`ValidationError`, and since `ActivityTypeAdapter.validate_python` has
no fallback, the entire activity was rejected, so affected bots silently
stopped responding.

Reported against 2.0.15, the first release containing #504. Confirmed to
affect customers, currently in 1:1 chats, with exposure growing as an
upstream server-side rollout ramps.

## Root cause
`channelData.app` is optional, so an absent `app` was always fine, but a
present but empty one wasn't. Auditing the model graph showed this isn't
unique to `app`: 98 sites exist where an optional parent points at a
child with required fields. Seven were reachable from a plain message
activity:
```
channelData.app={}       -> channelData.app.id            Field required
channelData.channel={}   -> channelData.channel.id        Field required
channelData.team={}      -> channelData.team.id           Field required
channelData.tenant={}    -> channelData.tenant.id         Field required
channelData.settings={}  -> channelData.settings.selectedChannel
attachments=[{}]         -> attachments.0.contentType
entities=[{}]            -> entities.0._unknown.type
```

## Change
Make these inbound-only fields optional: `AppInfo.id`, `ChannelInfo.id`,
`TeamInfo.id`, `TenantInfo.id`, `ChannelDataSettings.selected_channel`,
`Attachment.content_type`, `EntityBase.type`.

`MessageUpdateChannelData` and `MessageDeleteChannelData` both inherit
`ChannelData`, so they're covered without separate edits.

`EntityBase.type` is relaxed at the base rather than overridden on
`UnknownEntity`: every concrete entity already overrides `type` with a
`Literal` default, so `UnknownEntity` is the only class inheriting it
raw. This also means an entity type introduced after this SDK version no
longer drops the message.

## Tests
New `test_empty_inbound_objects.py` covering each empty-object case, an
unrecognized entity type, that populated values still parse, and that
absent `channelData` is unchanged.

## Validation
- 852 tests pass across `packages/api` + `packages/apps` 
- `ruff format`  / `ruff check`  clean
- pyright: zero net new errors

## Notes

This is the defensive half. The empty object is a service-side
wire-contract issue and has been raised with that team separately; this
change means the SDK degrades gracefully regardless.

(cherry picked from commit 5fcd8fa)