fix(server): cancel mapped request jobs when closing transport

[mikemikimike]

Fixes #922

Summary

• Cancel the coroutine job associated with each mapped Streamable HTTP request when the transport closes.
• Cancel replaced standalone GET stream jobs and explicitly closed SSE stream jobs as well.
• Preserve the current caller job so stateless request cleanup can finish normally.
• Add a regression test proving that closing the transport cancels a standalone GET SSE handler.

Test context

All Gradle commands were run in eclipse-temurin:21-jdk because the host default JDK is 8.

• :kotlin-sdk-server:jvmTest — passed, 229 tests, 0 failures, 0 errors.
• :kotlin-sdk-server:ktlintCheck — passed.
• :kotlin-sdk-server:detekt — passed.
• :kotlin-sdk-server:apiCheck — passed across the configured JVM, JS, and native API targets.
• :kotlin-sdk-server:jvmApiBuild — passed.
• git diff --check — passed.