[Internal] Tests: Refactors GatewayTests to use async/await instead of sync-over-async

[Copilot]

GatewayTests.cs contained 81 sync-over-async anti-patterns (.Result/.Wait()) across 16+ methods, risking deadlocks and masking exceptions via AggregateException wrapping.

Changes

New async infrastructure

• Added RetryAsync(Func<Task>) overload alongside existing Retry(Action) to support async retry lambdas

Helper methods → async

Old	New
CreateDocument (static Document)	async Task<Document>
CreateExecuteAndDeleteProcedure (with out param)	CreateExecuteAndDeleteProcedureCoreAsync returning tuple (async can't have out params)
CreateExecuteAndDeleteCosmosProcedure	CreateExecuteAndDeleteCosmosProcedureAsync
GetStoredProcedureExecutionResult	GetStoredProcedureExecutionResultAsync

void test methods → async Task

• ValidateTriggerCrud + its two [TestMethod] callers
• ValidateUserDefinedFunctionCrud + its two [TestMethod] callers
• ValidateUserDefinedFunctionTimeout, ValidateTriggersNameBased

Already-async methods: .Result/.Wait() → await

• ValidateStoredProcedureCrudAsync, ValidateTriggersInternal, ValidateSystemSprocInternal, ValidateStoredProceduresBlacklistingInternal, ValidateUserDefinedFunctions, ValidateStoredProcedureExecutionWithPartitionKey, ValidatePOCODocumentSerialization, ValidateCollectionQuotaTestsWithFailure

Exception handling fixed

Catch blocks that accessed AggregateException.InnerException (artifact of .Wait()/.Result) converted to catch the underlying DocumentClientException directly:

// Before
catch (AggregateException ex)
{
    Assert.AreEqual(HttpStatusCode.BadRequest, ((DocumentClientException)ex.InnerException).StatusCode);
}

// After
catch (DocumentClientException ex)
{
    Assert.AreEqual(HttpStatusCode.BadRequest, ex.StatusCode);
}

Collateral change

LinqGeneralBaselineTests.cs: updated caller of the renamed CreateExecuteAndDeleteCosmosProcedure to await GatewayTests.CreateExecuteAndDeleteCosmosProcedureAsync<int>(...).

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