[Internal] Tests: Adds DoNotParallelize to DistributedTransaction E2E tests (#5711)

## Summary

Adds `[DoNotParallelize]` attribute to `DistributedTransactionE2ETests`
to prevent concurrent test execution contention on the emulator.

## Root Cause

All 4 tests share a `TestInitialize` that creates a new container via
`BaseCosmosClientHelper.TestInit()` (which calls
`DeleteAllDatabasesAsync`). Concurrent test execution caused resource
contention on the emulator, resulting in consistent ~4% failure rate
across all 4 tests.

## Tests Fixed (95.82% pass rate each — 16 failures each in 30 days)
- `ValidateConflictResponseReturnsErrorStatus`
- `ValidateHappyPathRequestAndResponse`
- `ValidateMixedOperationsRequestStructure`
- `ValidateResponseDeserializesCorrectly`

## Impact
- **64 total flaky failures eliminated** (4 × 16)
- Split from #5643 for independent validation

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: NaluTripician

Microsoft.Azure.Cosmos/tests/Microsoft.Azure.Cosmos.EmulatorTests/DistributedTransaction/DistributedTransactionE2ETests.cs

@@ -18,6 +18,7 @@ namespace Microsoft.Azure.Cosmos.SDK.EmulatorTests
     using PartitionKey = Cosmos.PartitionKey;
 
     [TestClass]
+    [DoNotParallelize]
     public class DistributedTransactionE2ETests : BaseCosmosClientHelper
     {   
         private const string IdempotencyTokenHeader = HttpConstants.HttpHeaders.IdempotencyToken;