cloudflare-integration-tests: add shared streamed-span helpers to the runner

Blocks the other two sub-issues.

This package has no shared span assertion helpers. `suites/public-api/startSpan-streamed/test.ts` and `suites/tracing/ignoreSpans-streamed/test.ts` each define their own local `getSpanContainer(envelope)`, which will not scale to 80 ported files.

## What to do

Add helpers next to `runner.ts` and `expect.ts`, mirroring what the other packages already have:

* node uses `.expect({ span })` in `utils/runner/createRunner.ts`, which gives a `SerializedStreamedSpanContainer`
* browser uses `waitForStreamedSpan` / `waitForStreamedSpans` / `waitForStreamedSpanEnvelope` in `utils/spanUtils.ts`

At minimum this package needs: pull the span container out of an envelope, find a span by op or name, and collect spans across several envelopes, since streamed child spans do not all arrive in one envelope.

Port the two existing `-streamed` suites onto the new helpers in the same PR so the shape is proven before the bulk work starts.