feat(http): add SSE streaming support via Client.Stream

[sunfuze]

Summary

Add server-sent events (SSE) support to the http package.

• Client.Stream sends a request with Accept: text/event-stream and returns a pull-based *Stream. Request build errors, connection failures and non-200 responses (*ApiError) are returned synchronously from the call.
• Stream follows the bufio.Scanner / sql.Rows idiom (same shape as anthropic-sdk-go / openai-go SSE streams): Next() / Current() / Err() / Close() — no background goroutine, no locks, natural backpressure. Mid-stream read failures, timeouts and cancellation are reported by Err.
• Events() returns an iter.Seq[[]byte] (range-over-func) adapter yielding each event's data payload — multiple data: lines are joined with \n; comments/heartbeats and non-data fields (event:/id:/retry:) are skipped.
• Streaming uses a shallow copy of the configured http.Client with Timeout cleared, since http.Client.Timeout caps the entire request (headers + body read) and would kill a long-lived stream; the stream's lifetime is bounded by the caller's ctx instead.
• Refactor: extract request building/signing from Call into newRequest, shared by Call and Stream (no behavior change for Call).

Usage

stream, err := client.Stream(ctx, "GET", "/v1/xxx", nil, nil)
if err != nil {
    return err
}
defer stream.Close()

for data := range stream.Events() {
    // handle each event payload
}
if err := stream.Err(); err != nil {
    // read failure / timeout / cancellation
}

Test plan

• http/stream_test.go (httptest-based): multi-line data events, CRLF lines, comment/heartbeat skipping, trailing event without blank line, iterator + early break, non-200 → *ApiError, context timeout surfaced via Err(), connect failure, Close() idempotency
• GOWORK=off go test ./http/ -race — all green
• GOWORK=off go vet ./http/ — clean

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