Add Client.EventStream for reconnecting SSE through the pipeline

[OmarAlJarrah]

Summary

Adds Client.EventStream(ctx, req, opts...), a one-call reconnecting Server-Sent Events stream that runs through the client pipeline. Previously a caller had to write their own sse.ConnectFunc; now the common case — "stream this endpoint through my configured client" — is a single method that returns the same iter.Seq2[sse.Event, error] shape as the rest of the streaming APIs.

Each connection clones the request, sets Accept: text/event-stream (unless the caller set their own), replays the Last-Event-ID header after a mid-stream interruption, and sends it through Do — so auth, logging, tracing, and retry all run per connection. A non-2xx response or a transport failure on connect ends the stream with that error; a mid-stream interruption reconnects transparently. Requests with a body must be replayable (SSE is normally a bodyless GET).

Also adds the header.LastEventID constant.

Test plan

• Events flatten across reconnects in order; a terminal connect error ends the stream
• Last-Event-ID is replayed with the most recent event id on each reconnect
• Accept: text/event-stream is set on every connection
• A non-2xx connect ends the stream with an error and yields no events
• gofmt -l . clean, go vet ./... clean, go test -race ./... green across all packages

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