feat: implement Streamable HTTP HttpClient transport and improve error handling (#337)

- Add HttpClientStreamableHttpTransport implementing the 2025-03-26 MCP specification
- Add ResponseSubscribers utility for handling different HTTP response body types
- Refactor HttpClientSseClientTransport to use reactive streams instead of FlowSseClient
  - Remove FlowSseClient in favor of reactive stream patterns
  - Use Disposable-based connection management instead of CountDownLatch
  - Replace message endpoint discovery with Sinks.One approach
- Improve error handling by replacing doOnError/onErrorResume with onErrorComplete
- Add test coverage for new Streamable HTTP transport
- Add resiliency tests for connection handling and session invalidation

BREAKING CHANGE: FlowSseClient has been removed in favor of reactive streams

Signed-off-by: Christian Tzolov <christian.tzolov@broadcom.com>

Co-authored-by: Dariusz Jędrzejczyk <dariusz.jedrzejczyk@broadcom.com>

mcp-spring/mcp-spring-webmvc/src/test/resources/logback.xml

@@ -9,16 +9,16 @@
     </appender>
 
     <!-- Main MCP package -->
-    <logger name="io.modelcontextprotocol" level="DEBUG"/>
+    <logger name="io.modelcontextprotocol" level="INFO"/>
 
     <!-- Client packages -->
-    <logger name="io.modelcontextprotocol.client" level="DEBUG"/>
+    <logger name="io.modelcontextprotocol.client" level="INFO"/>
 
     <!-- Server transport package -->
-    <logger name="io.modelcontextprotocol.server.transport" level="DEBUG"/>
+    <logger name="io.modelcontextprotocol.server.transport" level="INFO"/>
 
     <!-- Spec package -->
-    <logger name="io.modelcontextprotocol.spec" level="DEBUG"/>
+    <logger name="io.modelcontextprotocol.spec" level="INFO"/>
 
     <!-- Root logger -->
     <root level="INFO">