feat: support suspending close callbacks

[original4422]

Summary

• change transport, protocol, session, and server close callbacks to suspending callbacks that are awaited by their close paths
• preserve callback registration order, AbstractTransport's exactly-once behavior, and its existing exception suppression while allowing cleanup to finish after transport-scope cancellation
• bridge WebSocket and server-SSE completion notifications through lifecycle-owned watcher jobs, including an explicit SSE close path that does not wait for the session Job to finish
• update public KDoc and API dumps for the intentional breaking API change

Testing

• ./gradlew :kotlin-sdk-core:jvmTest :kotlin-sdk-client:jvmTest :kotlin-sdk-server:jvmTest :kotlin-sdk-testing:jvmTest
• ./gradlew ktlintCheck apiCheck detekt
• ./gradlew apiDump
• compiled an out-of-repository consumer that calls delay() inside Transport.onClose

Fixes #919

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[original4422]

Hi @devcrocod, when you have a chance, would you mind reviewing this PR and letting me know whether any changes are needed? Thanks!