fix: Guard EventParser.lastEventId against a data race

[keelerm84]

getLastEventId() reads EventParser.lastEventId from arbitrary caller threads (via
EventSource.getLastEventId()), while it is written on the serialized delegate
queue during event dispatch. Reading a String concurrently with a write is
undefined behavior (a torn copy-on-write buffer), not merely a stale value; the
@unchecked Sendable delegate suppressed the compiler check.

Guard the field with an NSLock: lastEventId is now a computed property whose get
and set lock a private backing store, so the cross-thread read is safe regardless
of the writing thread. parse()/reset()/dispatch remain serialized on the delegate
queue and are unchanged. Mark EventParser @unchecked Sendable to document that
split (serialized mutation + lock-guarded cross-thread read).

Adds a concurrency test that hammers getLastEventId() from one task while another
drives events, asserting correctness under load. Run under --sanitize=thread to
catch a regression (TSan is unavailable on the Linux toolchain here; the macOS CI
leg is TSan-capable).

---

Note

Medium Risk
Touches hot-path SSE state read from arbitrary threads; the lock is narrow but incorrect locking could still affect reconnect Last-Event-Id behavior under load.

Overview
Fixes a data race on EventParser.lastEventId: getLastEventId() can run on caller threads (via EventSource) while dispatch updates the field on the serialized delegate queue. Concurrent String read/write is undefined behavior, not just stale values.

lastEventId is now backed by _lastEventId behind an NSLock via a computed property; parse/reset/dispatch stay on the delegate queue. EventParser is @unchecked Sendable with comments describing that split.

Adds getLastEventIdIsSafeDuringConcurrentParsing (writer parses events, reader hammers getLastEventId()). CI gains test-thread-sanitizer on Ubuntu (swift:6.1) running swift test --sanitize=thread with TSAN_OPTIONS=halt_on_error=1 so races fail the job.

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