Improve error handling for message enqueue failure

[codeboyzhou]

• Update sendMessage method to provide more detailed error information
• Remove unnecessary imports and improve code readability

Motivation and Context

While debugging an MCP server in STDIO mode, I encountered RuntimeException: Failed to enqueue message. The exception stack trace pointed to StdioMcpSessionTransport#sendMessage() but without more error details, I can only see that outboundSink.tryEmitNext(message).isSuccess() actually return false but there is no more helpful info to locate the root cause what results in this false. I think the return value of tryEmitNext(message) is needed to be included in the error message to make it more clear.

How Has This Been Tested?

Yes, I tested this in my local MCP server example application.

Breaking Changes

No breaking changes

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

No additional context

[codeboyzhou]

This PR is not very important, so consider closing it temporarily to avoid the frequent Merge branch 'modelcontextprotocol:main' into main action when I sync fork from upstream every time, perhaps another new PR will be committed in the future.