(closes #18360)

## Problem

In getsentry/sentry-mcp#652 we had to manually
wait for the flush to ensure that async operations completed before the
response was sent.

This was caused by the MCP server streaming data after the actual
request has been finished. When using the Cloudflare SDK with MCP
servers (or other streaming APIs), spans created during the streaming
phase were not being flushed properly because the `flush()` method would
complete before these async spans finished.

## Solution

During the request, when a span is created for the streaming call, we
now register listeners on the CloudflareClient for `spanStart` and
`spanEnd` events. The client tracks all pending spans and waits for them
to complete before flushing.

Specifically:
- Added `_pendingSpans` Set to track active span IDs
- Created a promise-based mechanism (`_spanCompletionPromise`) to wait
for all spans to complete
- Modified the `flush()` method to wait for pending spans before calling
`super.flush()`
- Added a timeout (default 5 seconds) to prevent indefinite waiting

## Test

Added a comprehensive E2E test application (`cloudflare-mcp`) that:
- Sets up a Cloudflare Worker with an MCP server
- Registers a tool that creates spans during execution
- Verifies that both the HTTP request span and the MCP tool call span
are properly sent to Sentry
- Validates span relationships (parent/child) and trace IDs

The test confirms that spans are now flushed correctly without manual
intervention.

In #652 we had to manually add the flush, which should actually be done
by the Cloudflare SDK. This has now landed in `10.28.0` (via [this
PR](getsentry/sentry-javascript#18334)) and no
manual flushing has to be done anymore.

Just to be sure sure I verified on top if everything has been sent
locally. This is a trace from my local deployment:

https://sentry-sdks.sentry.io/explore/traces/trace/434f1b1e8655444c875bce5c66645376/?fov=0%2C291&node=span-84a7a24ae593ee49&project=4510380305022976&source=traces&statsPeriod=15m&targetId=a505468e10176600&timestamp=1764689933

I recommend to review this PR [without whitespaces
enabled](https://github.com/getsentry/sentry-mcp/pull/663/files?w=1).