fix(server): preserve id on duplicate initialize errors

[jstar0]

Summary

Fixes #866.

Changes

• Preserve the parsed JSON-RPC request id when Streamable HTTP rejects a duplicate initialize request.
• Keep pre-parse and transport-level rejection paths unchanged when the request id is not available.
• Extend the duplicate-initialize regression test to assert the error response id as well as the HTTP status.

Verification

./gradlew :kotlin-sdk-server:jvmTest --tests 'io.modelcontextprotocol.kotlin.sdk.server.StreamableHttpServerTransportTest.second initialization request returns JSON-RPC error with request id'
./gradlew :kotlin-sdk-server:jvmTest --tests 'io.modelcontextprotocol.kotlin.sdk.server.StreamableHttpServerTransportTest'
./gradlew :kotlin-sdk-server:jvmTest
./gradlew :kotlin-sdk-server:ktlintCheck :kotlin-sdk-server:detekt

[devcrocod]

lgtm