feat!: AsyncSequence-first public API

[keelerm84]

Phase 2 of the Swift 6 modernization (SDK-2607). Replaces the push-based
EventHandler callback with an AsyncStream of events that consumers iterate with
for await. This is the surface the swift-core streaming FDv2Synchronizer
(Phase 3) needs; an AsyncStream maps almost 1:1 onto its next() async.

BREAKING CHANGE: EventHandler is no longer public and EventSource.Config no
longer takes a handler. Consume EventSource.events instead.

Public API:

• Add EventSourceEvent (opened / closed / message / comment / error).
• EventSource.events exposes an AsyncStream<EventSourceEvent>; Config.init
  drops the handler parameter (now just init(url:)).
• Errors are delivered as .error values, not stream terminations: the client
  keeps retrying with backoff and the stream continues, ending only on stop()
  (or consumer cancellation). Lifecycle stays explicit via start()/stop().

Internal:

• EventHandler becomes internal; EventParser still drives it. A private
  ContinuationEventHandler forwards the delegate's and parser's callbacks into
  the stream's continuation. The URLSessionDataDelegate data path is unchanged
  (no URLSession.bytes, which is unavailable on Linux/Windows Foundation).
• The delegate finishes the continuation at terminal shutdown; the consumer
  cancelling iteration tears the connection down via onTermination.

Tests / contract service:

• Network tests become async and observe events via a non-blocking poll-based
  AsyncSink drained from the stream (no thread blocking on the cooperative
  executor); the suite stays .serialized for the global URLProtocol state. The
  parser tests keep the synchronous EventSink. Buffering is .unbounded.
• Contract test service consumes events in a Task and forwards to the callback
  URL instead of implementing EventHandler.

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Note

High Risk
Breaking public API and changed connection/stream lifecycle (unbounded buffering, single consumer, cancellation teardown) affect all integrators and long-lived SSE consumers.

Overview
Breaking change: public EventHandler is removed and EventSource.Config no longer takes a handler—callers use for await on EventSource.events instead.

Adds EventSourceEvent (opened, closed, message, comment, error). Network errors are .error values, not stream termination; the client keeps reconnecting until stop() (or consumer cancellation). stop() finishes the stream; cancelling the consumer tears the connection down via onTermination; deinit also stops if never shut down. Reconnect scheduling is blocked once shutdown.

Internally, ContinuationEventHandler still feeds the existing parser/delegate path into the stream continuation (URLSession delegate unchanged).

Tests move to async observation via EventCollector / poll-based AsyncSink; the contract test service forwards the stream with CallbackForwarder in a Task.

^{Reviewed by Cursor Bugbot for commit 52b7ac5. Bugbot is set up for automated code reviews on this repo. Configure here.}

[tanderson-ld]

In old code, error handler was invoked even for unrecoverable errors. Is this an intentional behavior change?