Settle pending requests when the SSE response leg ends early

[ege-arhan]

With StreamableHTTP, a POST can get a text/event-stream body that hits EOF or errors before the matching JSON-RPC response arrives. The transport already notices this and fires onRequestStreamEnd, but Protocol.request never passed that callback into transport.send, so the pending request just sat there until the outer timeout.

This threads the callback through: the caller's onRequestStreamEnd still runs, and the request now rejects promptly with ConnectionClosed instead of waiting for a timeout. Verified with tsc --noEmit clean and the 49 protocol tests passing.

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