## 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
```bash
./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
0221b8db3d