fix: improve streamable HTTP session reinitialization (#459)

Implements the MCP spec guidelines for streamable HTTP (re)initialization:
- Server MAY terminate session and MUST respond with HTTP 404 for terminated session IDs
- Client MUST start new session when receiving HTTP 404 for requests with session ID

Changes:
- Replace generic McpError with McpTransportException for transport-layer errors
- Only throw McpTransportSessionNotFoundException when session ID is present in request
  (per spec: 404 with session ID means session terminated, without means general error)
- Enhance error messages with more context (status codes, response events)
- Use RuntimeException for non-transport specific SSE endpoint failures
- Ensure consistent error handling across HTTP client transports

- Improve error handling with standard Java exceptions.
  Replace generic McpError with appropriate standard exceptions:
  - Use IllegalArgumentException for invalid input parameters
  - Use IllegalStateException for state-related issues
  - Use RuntimeException wrapper for initialization failures
  - Use McpError.builder() with proper error codes for protocol errors

Fixes #459

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

mcp-spring/mcp-spring-webmvc/src/test/java/io/modelcontextprotocol/server/WebMvcSseIntegrationTests.java

@@ -11,6 +11,7 @@ import org.apache.catalina.LifecycleException;
 import org.apache.catalina.LifecycleState;
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Timeout;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.web.reactive.function.client.WebClient;
@@ -29,6 +30,7 @@ import io.modelcontextprotocol.server.McpServer.SingleSessionSyncSpecification;
 import io.modelcontextprotocol.server.transport.WebMvcSseServerTransportProvider;
 import reactor.core.scheduler.Schedulers;
 
+@Timeout(15)
 class WebMvcSseIntegrationTests extends AbstractMcpClientServerIntegrationTests {
 
 	private static final int PORT = TestUtil.findAvailablePort();

mcp-spring/mcp-spring-webmvc/src/test/java/io/modelcontextprotocol/server/WebMvcStatelessIntegrationTests.java

@@ -11,6 +11,7 @@ import org.apache.catalina.LifecycleException;
 import org.apache.catalina.LifecycleState;
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Timeout;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.web.reactive.function.client.WebClient;
@@ -29,6 +30,7 @@ import io.modelcontextprotocol.server.McpServer.StatelessSyncSpecification;
 import io.modelcontextprotocol.server.transport.WebMvcStatelessServerTransport;
 import reactor.core.scheduler.Schedulers;
 
+@Timeout(15)
 class WebMvcStatelessIntegrationTests extends AbstractStatelessIntegrationTests {
 
 	private static final int PORT = TestUtil.findAvailablePort();

mcp-spring/mcp-spring-webmvc/src/test/java/io/modelcontextprotocol/server/WebMvcStreamableIntegrationTests.java

@@ -11,6 +11,7 @@ import org.apache.catalina.LifecycleException;
 import org.apache.catalina.LifecycleState;
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Timeout;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.web.reactive.function.client.WebClient;
@@ -29,6 +30,7 @@ import io.modelcontextprotocol.server.McpServer.SyncSpecification;
 import io.modelcontextprotocol.server.transport.WebMvcStreamableServerTransportProvider;
 import reactor.core.scheduler.Schedulers;
 
+@Timeout(15)
 class WebMvcStreamableIntegrationTests extends AbstractMcpClientServerIntegrationTests {
 
 	private static final int PORT = TestUtil.findAvailablePort();