fix(client): close stdio read sources during shutdown

[jstar0]

Summary

Fixes StdioClientTransport.close() so shutdown does not hang when the stdin read loop is blocked waiting for input.

Root Cause / Context

StdioClientTransport.closeResources() stopped the transport coroutine scope and then waited for the scope job to finish. On the JVM, a Source.readAtMostTo() call backed by a blocking Java stream does not necessarily return when the coroutine is cancelled. If the read loop stayed blocked, close() could wait indefinitely.

Changes

• Close the stdin source during stdio client shutdown before waiting for transport coroutines to finish.
• Also close the optional stderr source during shutdown.
• Keep source-close failures non-fatal and log them at debug level.
• Add a JVM regression test with a RawSource that blocks in readAtMostTo() until close() is called.
• Assert the close path completes, emits onClose, and does not invoke onError.

Scope / Risk

This is limited to StdioClientTransport shutdown behavior. It does not change the public API or server transports. The regression coverage is JVM-focused because the reported hang is caused by a blocking Java stream read.

Verification

./gradlew :integration-test:jvmTest --tests 'io.modelcontextprotocol.kotlin.sdk.client.StdioClientTransportTest'
./gradlew :kotlin-sdk-client:jvmTest
./gradlew :kotlin-sdk-client:ktlintCheck :integration-test:ktlintCheck :kotlin-sdk-client:detekt :integration-test:detekt :kotlin-sdk-client:apiCheck

Closes #514