Document MongoDB single-node replica set mode in hosting integration guide

[Copilot]

The MongoDB hosting docs did not cover the new single-node replica set support added in Aspire, leaving gaps for users who need transactions or change streams. This updates the MongoDB hosting integration page to explain when to enable replica set mode and what behavior changes to expect.

• Replica set guidance and discoverability

  • Added an intro-level pointer to a new Enable replica set mode section.
  • Added clear guidance on when to use replica sets (transactions/change streams) vs. when to keep standalone mode (routine CRUD).

• API usage examples (copy/paste)

  • Added AppHost examples for both C# and TypeScript showing replica set enablement.

  var mongo = builder.AddMongoDB("mongo")
      .WithReplicaSet();

  const mongo = await builder.addMongoDB("mongo");
  await mongo.withReplicaSet();

• Connection and runtime behavior notes

  • Documented that WithReplicaSet() defaults to rs0 and accepts an optional string name override.
  • Added a connection string example showing directConnection=true when replica set mode is enabled.
  • Added a note clarifying that Aspire starts the local MongoDB container as a one-node replica set and initializes it at startup.

[Copilot]

Pull request overview

Updates the MongoDB hosting integration guide to document Aspire’s single-node replica set support, clarifying when and how to enable replica set mode and what connection/runtime behavior to expect.

Changes:

• Adds an “Enable replica set mode” section with guidance for transactions/change streams vs. standalone CRUD use.
• Adds C# and TypeScript AppHost examples showing how to enable replica set mode.
• Documents default replica set naming and provides a connection string example for replica set mode.

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