A stateless streamable-HTTP request is one-shot (no session, no resumption),
so if the client drops the response before the handler finishes, the request
is terminal and should be cancelled. Previously the handler kept running with
its `RequestContext::ct` never firing, so long-running or destructive tools
could not observe a client disconnect.
Give each stateless request its own cancellation token via
`serve_directly_with_ct` and cancel it when the client disconnects. This covers
both stateless paths: `serve_negotiated_request_directly` (per-request version
negotiation) and the non-negotiated path. In each:
- A disconnect while the handler is still producing its first message cancels
it. The guard is disarmed once the handler emits anything, so a normal
response is never cancelled.
- The SSE response stream is wrapped in a guard that cancels the handler if the
stream is dropped before it ends naturally.
- When a negotiated request replies directly (JSON mode, or a non-OK status),
the receiver is dropped, so a still-running handler is cancelled rather than
left emitting into a closed channel. Without this its terminal send fails
before adding the termination permit and the serve loop parks forever.
Stateful (resumable) mode is intentionally left unchanged: there a disconnect
may be recovered via `Last-Event-ID`, so cancelling on disconnect would break
resumption.
Adds a regression test covering both stateless sub-modes (SSE and JSON).
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