… message (#419)

Fixes #418

## Summary

`HttpStream.close()` could send the final stream message while
`_flush()` was still mid-await, causing the final message to arrive
before in-flight chunks finished sending.

This is the Python parity of
[microsoft/teams.ts#553](microsoft/teams.ts#553)
(Bug 2 only — Bug 1 from the TS PR does not affect Python; `close()`
already calls `with_channel_data()` before `add_stream_final()`).

## Root cause

`_wait_for_id_and_queue()` polled:

```python
while (self._queue or not self._id) and not self._canceled:
```

This checks the queue and `_id` but not whether `_flush()` is currently
holding `self._lock`. After the inner `while self._queue` loop drains
the queue, `_flush()` awaits `_send_activity(...)` calls under the lock.
During those awaits:

- `self._queue` is empty (drained)
- `self._id` is set (after the first chunk)
- `self._lock` is still held

The predicate becomes false, the wait exits, and `close()` proceeds to
send the final activity — racing the in-flight chunk.

## Fix

Add `self._lock.locked()` to the wait predicate. The lock primitive is
already used in `close()`'s early-return guard at line 157, so this is
consistent with existing patterns.

## Test plan

- [x] Added `test_close_waits_for_flush_to_complete` — holds
`stream._lock` manually, asserts `close()` does not send the final
message until the lock is released.
- [x] All existing http-stream tests still pass (14/14).
- [x] Full apps test suite passes (271 passed).
- [x] `poe check` passes (ruff format + lint).

Merges main into release and sets version to 2.0.10.

## Commits since last release

- 7147cd8 fix(apps): support AAD v1 issuers in token validation (#556)
- eb8037e address model gaps (#525)
- ac21af6 fix(http-stream): resolve race condition in close() and
streamType override in final message (#553)
- 6b0da8a MCP Server Example on MCP SDK (#534)
- a749172 chore: bump version to 2.0.10-preview (#555)

)

Fixes #473

## Summary

Two bugs in the streaming path. Parity of
[microsoft/teams.ts#553](microsoft/teams.ts#553),
but Bug 1 manifests via a different mechanism in C# (`??=`) than in TS.

### Bug 1: `AddStreamFinal()` cannot override existing
`ChannelData.StreamType`

`MessageActivity.AddStreamFinal()` used null-coalescing assignment:

```csharp
ChannelData.StreamType ??= StreamType.Final;
```

In the streaming `Close()` path:

1. `Stream.Update("Thinking...")` accumulates `StreamType.Informative`
into `_channelData`.
2. `Close()` calls `WithData(_channelData)`, populating
`activity.ChannelData.StreamType = Informative`.
3. `AddStreamFinal()` runs `??=` against `Informative` → **no-op**. The
wire `channelData.streamType` stays as `"informative"` on the final
message, even though the `streaminfo` entity correctly has `"final"`.

**Fix:** direct assignment. The method's contract per its name is to
mark the activity as a final stream message, so it must be able to
override prior values.

> **Note:** This is a minor public-API behavior change. Any caller that
pre-set `ChannelData.StreamType` and then called `AddStreamFinal()`
expecting their value to persist will now have it overridden. The
method's name and contract make the override semantic the obvious one;
we judged this acceptable.

### Bug 2: Race condition in `Stream.Close()`

`Close()`'s wait loop checked `_id` and `_queue` but not whether
`Flush()` was holding `_lock`. During a flush mid-await (queue drained,
`SendActivity()` awaits pending), `Close()` would exit the loop and send
the final activity, racing the in-flight chunk.

**Fix:** also wait while `_lock.CurrentCount == 0` (semaphore held).
This is the same primitive already used in `Close()`'s early-return
guard on line 88.

## Changes

- `Libraries/Microsoft.Teams.Api/Activities/Message/MessageActivity.cs`
— `AddStreamFinal()` direct assignment.
-
`Libraries/Microsoft.Teams.Plugins/Microsoft.Teams.Plugins.AspNetCore/AspNetCorePlugin.Stream.cs`
— `Close()` lock-aware wait.
- Tests in `MessageActivityTests.cs` and
`AspNetCorePluginStreamTests.cs`.

## Test plan

- [x] `AddStreamFinal_OverridesExistingStreamType` — `MessageActivity`
with `StreamType.Informative` pre-set, `AddStreamFinal()` overrides to
`Final`.
- [x]
`Stream_Close_FinalMessageHasStreamTypeFinal_AfterInformativeUpdate` —
full Update→Emit→Close flow asserts final activity has
`StreamType.Final` on both `ChannelData` and the `streaminfo` entity.
- [x] `Stream_Close_WaitsForInFlightFlushToComplete` — uses a
controllable `Send` delegate to block the second flush mid-await;
asserts `Close()` does not progress until the lock releases.
- [x] All existing `MessageActivityTests` (21 passed) and
`AspNetCorePluginStreamTests` (6 passed) still pass on net10.0. (net8.0
testhost not installed locally; CI will cover.)
- [x] Full solution builds clean.