[rc2] Fix race condition in SingletonCosmosClientWrapper.Client

[Copilot]

Fixes #36714

Description
The SingletonCosmosClientWrapper.Client property was using non-thread-safe lazy initialization, which caused multiple CosmosClient instances to be created under concurrent access. This led to resource contention, 100% CPU usage, and 503 errors. The fix moves CosmosClient initialization to the constructor and simplifies the conditional logic for better maintainability.

Customer impact
In high-throughput multi-threaded applications this caused resource contention, 100% CPU usage, and 503 errors. There isn't a viable workaround.

How found
Customer reported. This likely affects many EF Cosmos users to some degree but hasn't been reported before as it usually doesn't manifest itself in a reproducible manner.

Regression
No.

Testing
Test added

Risk
Low. Localized change moving from lazy initialization to constructor initialization, which is more straightforward and eliminates thread safety concerns entirely.

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[AndriySvyryd]

@copilot Rewrite the PR summary using this template:

Fixes #{number}

**Description**
{Issue description}

**Customer impact**
{How does the reported issue affects customer's app? Are there workarounds?}

**How found**
{Was it customer reported or found during verification? How many customers are affected?}

**Regression**
{Is it a regression from a released version? Which one?}

**Testing**
{How the changes were tested}

**Risk**
{Low/Medium/High, argument why}

[Copilot]

@copilot Rewrite the PR summary using this template:

...

Done. Rewrote the PR description using the requested template with sections for Description, Customer impact, How found, Regression, Testing, and Risk.

[AndriySvyryd]

@copilot Use the default emulator URI and key in the UseCosmos calls in PartitionKeyBuilderExtensionsTest.cs and CosmosConventionSetBuilder.cs

[Copilot]

@copilot Use the default emulator URI and key in the UseCosmos calls in PartitionKeyBuilderExtensionsTest.cs and CosmosConventionSetBuilder.cs

Done. Updated both files to use the default Cosmos emulator URI (https://localhost:8081) and key (C2y6yDjf5/R+ob0N8A7Cgv30VRDJIWEHLM+4QDU5DE2nQ9nDuVTqobD4b8mGGyPMbIZnqyMsEcaGQy67XIw/Jw==) in the UseCosmos calls. Commit 568b2df

[AndriySvyryd]

@copilot Update Throws_for_missing_connection_info to call context.GetService<IDatabaseCreator>() inside of Assert.ThrowsAsync lambda

[Copilot]

@copilot Update Throws_for_missing_connection_info to call context.GetService<IDatabaseCreator>() inside of Assert.ThrowsAsync lambda

Done. Updated the test to call context.GetService<IDatabaseCreator>() inside the Assert.ThrowsAsync lambda instead of declaring it as a separate variable. This ensures the exception is caught when the service is accessed. Commit e7c3a56

[artl93]

RC2