fix(audience-http): propagate caller cancellation from SendBatchAsync

Addresses PR #701 review from @nattb8.

The catch-filter branch for caller cancellation used to silently swallow
the exception and let the method return `true` — its normal "batch sent,
ask me again" signal. FlushAsync's send loop takes that return value at
face value and immediately re-enters on the same cancelled token.
HttpClient throws on entry (token still cancelled), the same branch
swallows it, `true` is returned again. The batch is never deleted on
this path, so ReadBatch keeps handing back the same events — a tight
infinite loop.

Rethrow instead. The caller's send loop exits via the exception; no
behaviour change for HttpClient's internal timeout path (still filtered
out by the `when (ct.IsCancellationRequested)` guard) so timeouts still
trigger backoff.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: ImmutableJeffrey

src/Packages/Audience/Runtime/Transport/HttpTransport.cs

@@ -131,10 +131,15 @@ internal async Task<bool> SendBatchAsync(CancellationToken ct = default)
             }
             catch (OperationCanceledException) when (ct.IsCancellationRequested)
             {
-                // Caller cancelled the token (e.g. on shutdown). Events stay on
-                // disk, no failure recorded. HttpClient timeouts throw the same
-                // exception but without ct.IsCancellationRequested set, so they
-                // fall through to the Exception branch below and trigger backoff.
+                // Caller cancelled the token (e.g. on shutdown). Events stay
+                // on disk, no failure recorded. Rethrow so the caller's send
+                // loop exits — swallowing here returns `true`, and the loop
+                // would re-enter on the same cancelled token and spin because
+                // the batch is still on disk. HttpClient timeouts throw the
+                // same exception but without ct.IsCancellationRequested set,
+                // so they fall through to the Exception branch below and
+                // trigger backoff.
+                throw;
             }
             catch (Exception ex)
             {