fix(core): keep resumption options off cancellation notifications - #2649

Fixes #2646.

Root cause

When a legacy-era request was aborted, Protocol reused the original request's full transport options for notifications/cancelled. If that request carried a resumptionToken, StreamableHTTPClientTransport interpreted the cancellation send as a resume operation, swallowed the notification, and opened a resumed GET instead of posting the cancellation.

Fix

• Keep relatedRequestId on the cancellation send so request correlation is preserved.
• Do not pass resumptionToken or onresumptiontoken to the cancellation notification.
• Add a regression test that records the transport options for the cancellation send.
• Add a patch changeset for @modelcontextprotocol/core.

Validation

• Regression test fails against the previous implementation and passes with this change.
• pnpm --filter @modelcontextprotocol/core-internal test: 69 files, 1,434 tests passed.
• Monorepo build passed via the pre-push hook.
• Monorepo typecheck passed via the pre-push hook.
• pnpm lint:all passed.
• Changeset status and formatting checks passed.