fix(client): Fixes a bug in McpClientSession.sendRequest()

[logan-querypie]

Motivation and Context

Fixes a bug in McpClientSession.sendRequest() where it waited until timeout when receiving error responses such as 4xx or 5xx.
Returning failedFuture() when an error is detected in HttpClientSseClientTransport.sendMessage() resolves the issue.

How Has This Been Tested?

Our project’s server uses this SDK to relay MCP communication, and we’ve confirmed that it works well in that environment.

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

[tzolov]

Thanks for catching that @logan-querypie!

As we implement the streamable HTTP client transport, we're also consolidating/improving the SSE transport.
Draft: #337

I applied your approach here:

java-sdk/mcp/src/main/java/io/modelcontextprotocol/client/transport/HttpClientStreamableHttpTransport.java

Lines 236 to 239 in fef3bdb

	.exceptionallyCompose(e -> {
	logger.warn("Error sending message", e);
	sseSink.error(e);
	return CompletableFuture.failedFuture(e);

Feel free to review the draft PR. Any fixes and improvements are appreciated.
I will keep this PR open until the #337 is merged

[tzolov]

The #337 is merged and should addresses this issue as well.
I'm closing the PR, but please feel free to re-open it if the issue is still present.