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GH-44360: [C#] Fix Flight DoExchange incompatibility with C++ implementation (#44424)

### Rationale for this change

See #44360

### What changes are included in this PR?

* Adds a new integration test to allow testing `do_exchange` between C++/Python and .NET.
* Updates the Flight stream reader to handle when a descriptor is sent in the first message without any schema.

### Are these changes tested?

* Yes, using the new integration test.

### Are there any user-facing changes?

No
* GitHub Issue: #44360

Authored-by: Adam Reeve <adreeve@gmail.com>
Signed-off-by: Curt Hagenlocher <curt@hagenlocher.org>