fix(server): replay late initialization callbacks

[mikemikimike]

Summary

• Replay ServerSession.onInitialized callbacks registered after notifications/initialized.
• Preserve registration order for callbacks registered before initialization and make the state transition race-safe.
• Add a handshake-level regression test and document synchronous replay behavior.

Fixes #920

Validation

• :integration-test:jvmTest --tests "io.modelcontextprotocol.kotlin.sdk.server.ServerSessionInitializeTest" — passed
• :kotlin-sdk-server:jvmTest — passed
• :kotlin-sdk-server:ktlintCheck :integration-test:ktlintCheck — passed
• :kotlin-sdk-server:detekt :integration-test:detekt — passed
• :kotlin-sdk-server:jvmApiCheck — passed
• git diff --check — passed