examples: add prewarm and SSE examples

[Rememorio]

Summary

• Add a prewarm example that uses client.InitializableClient, client.WithPreWarm, and the existing generated helloworld proxy.
• Add a minimal SSE example using the public HTTP client SSEHandler and http.WriteSSE APIs.
• Document precool readiness checks in the server user guide without adding new framework APIs.

Compatibility

• Uses only existing public APIs.
• Does not add framework behavior, new generated stub requirements, or transport plugin dependencies.
• Keeps internal metrics and connection counters out of the examples.

Tests

• go test ./features/prewarm/... ./features/sse/... -count=1 from examples/
• go test ./features/precool/... -count=1 from examples/
• go test ./... -count=1 from examples/
• go test ./... -count=1
• Ran the prewarm example server/client locally
• Ran the SSE example server/client locally
• git diff --cached --check
• Scanned changed files for internal-only links and local paths

[WineChord]

LGTM