Add SSE/streaming response documentation and tests

[jeremydmiller]

Summary

• Add streaming endpoints using Results.Stream() with text/event-stream and text/plain content types
• Add integration tests verifying SSE and plain text streaming work with Wolverine.HTTP endpoints
• Add documentation page explaining how to use streaming/SSE with Wolverine (uses standard ASP.NET Core, no custom Wolverine code needed)

Test plan

• 2 integration tests pass (SSE events, plain text streaming)

Closes #2403

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