Enable tracing for RabbitMQ Component (v7)

[Copilot]

RabbitMQ.Client v7 ships built-in OpenTelemetry activity sources (RabbitMQ.Client.Publisher and RabbitMQ.Client.Subscriber). The code to register these sources via AddSource("RabbitMQ.Client.*") was already in place for v7 but lacked conformance tests and documentation.

Changes

ConformanceTests.cs

• Set ActivitySourceName conditionally: "Aspire.RabbitMQ.Client" for v6, "RabbitMQ.Client.Publisher" for v7
• Added TracingEnablesTheRightActivitySource / TracingEnablesTheRightActivitySource_Keyed tests (v7 only via #if !RABBITMQ_V6), following the same pattern as Npgsql/MySqlConnector
• Custom RunActivitySourceTest instead of base class ActivitySourceTest: IConnection creation generates connection activities before TriggerActivity is called, which breaks the base class Assert.Empty check — the custom method clears those before asserting on publisher activities
• Refactored PopulateConfiguration to use a stored ConnectionString property (required to pass the connection string into the RemoteExecutor subprocess)

README.md

• Added Observability section documenting the three activity sources (Aspire.RabbitMQ.Client, RabbitMQ.Client.Publisher, RabbitMQ.Client.Subscriber)
• Fixed Usage example: _AppHost.cs_ → _Program.cs_

Checklist

• Is this feature complete?
  • Yes. Ready to ship.
  • No. Follow-up changes expected.
• Are you including unit tests for the changes and scenario tests if relevant?
  • Yes
  • No
• Did you add public API?
  • Yes
  • No
• Does the change make any security assumptions or guarantees?
  • Yes
  • No
• Does the change require an update in our Aspire docs?
  • Yes
  • No

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[maddymontaquila]

@eerhardt i thought this might be an easy one but feels like slop lol. are we up to date w rabbitmq and does it have tracing now?