fix: cancel a Streamable HTTP call by closing its stream (#2140)

The Cancel button POSTed `notifications/cancelled` on every transport. On
a 2026-07-28 Streamable HTTP connection that is the wrong signal — the
spec makes closing the request's SSE response stream the cancellation
signal there and says the notification is neither required nor expected —
so a spec-compliant server acknowledged it 202 and dropped it while the
tool ran to completion.

The SDK already implements the fork, off `transport.hasPerRequestStream`.
Every Inspector connection is wrapped in `MessageTrackingTransport`, which
did not forward that flag, so the SDK read `undefined` and took the stdio
branch on every client — the CLI and TUI included, whose real
`StreamableHTTPClientTransport` advertises it correctly.

The web client needed three more links: its browser transport answers for
the upstream transport that lives on the backend, so it advertises the
flag for `streamable-http` and applies the SDK's `requestSignal` to its
`POST /api/mcp/send`; and `/api/mcp/send`, held open for the whole call,
forwards the browser's disconnect to the upstream `transport.send`.

The test fixture's `toWebRequest` built a Web Request with no `signal`, so
no fixture server could observe a disconnect at all — without that, a test
asserting the cancel reaches the server passes vacuously.

stdio and SSE keep `notifications/cancelled`: they multiplex every request
over one shared channel and have no per-request stream to close. So does
the legacy era, which predates the mechanism.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013hzwzS8UvBsr4yZm7o7sev
Signed-off-by: cliffhall <cliff@futurescale.com>

test-servers/configs/cancellation-modern-http.json

@@ -0,0 +1,12 @@
+{
+  "serverInfo": {
+    "name": "cancellation-showcase",
+    "version": "1.0.0"
+  },
+  "tools": [{ "preset": "slow_task" }, { "preset": "echo" }],
+  "transport": {
+    "type": "streamable-http",
+    "port": 6606,
+    "modern": true
+  }
+}