Use rescue StandardError for stream close

## Motivation and Context

Replace bare `rescue` in stream cleanup with `rescue StandardError`
so signals and exits are not swallowed.

## How Has This Been Tested?

Add regression tests for `cleanup_session_unsafe` and `reap_expired_sessions`.

## Breaking Changes

None.

Author: koic

lib/mcp/server/transports/streamable_http_transport.rb

@@ -150,8 +150,8 @@ def reap_expired_sessions
             # removed from `@sessions` above, so other threads will not find them
             # and will not attempt to close the same stream.
             stream.close
-          rescue
-            nil
+          rescue StandardError
+            # Ignore close-related errors from already closed/broken streams.
           end
         end
 
@@ -239,8 +239,8 @@ def cleanup_session_unsafe(session_id)
 
           begin
             session[:stream]&.close
-          rescue
-            nil
+          rescue StandardError
+            # Ignore close-related errors from already closed/broken streams.
           end
           @sessions.delete(session_id)
         end

test/mcp/server/transports/streamable_http_transport_test.rb

@@ -579,6 +579,58 @@ class StreamableHTTPTransportTest < ActiveSupport::TestCase
           assert_equal({}, @transport.instance_variable_get(:@sessions))
         end
 
+        test "reap_expired_sessions rescues StandardError from stream close" do
+          transport = StreamableHTTPTransport.new(@server, session_idle_timeout: 0.01)
+
+          init_request = create_rack_request(
+            "POST",
+            "/",
+            { "CONTENT_TYPE" => "application/json" },
+            { jsonrpc: "2.0", method: "initialize", id: "init" }.to_json,
+          )
+          init_response = transport.handle_request(init_request)
+          session_id = init_response[1]["Mcp-Session-Id"]
+
+          # Replace the stream with one that raises `IOError` on close.
+          error_stream = Object.new
+          error_stream.define_singleton_method(:close) { raise IOError, "already closed" }
+          transport.instance_variable_get(:@sessions)[session_id][:stream] = error_stream
+
+          sleep(0.02) # Wait for session to expire.
+
+          # Should not raise because `StandardError` is rescued.
+          transport.send(:reap_expired_sessions)
+          assert_empty(transport.instance_variable_get(:@sessions))
+        ensure
+          transport.close
+        end
+
+        test "cleanup_session_unsafe rescues StandardError from stream close" do
+          init_request = create_rack_request(
+            "POST",
+            "/",
+            { "CONTENT_TYPE" => "application/json" },
+            { jsonrpc: "2.0", method: "initialize", id: "init" }.to_json,
+          )
+          init_response = @transport.handle_request(init_request)
+          session_id = init_response[1]["Mcp-Session-Id"]
+
+          # Replace the stream with one that raises `IOError` on close.
+          error_stream = Object.new
+          error_stream.define_singleton_method(:close) { raise IOError, "already closed" }
+          @transport.instance_variable_get(:@sessions)[session_id][:stream] = error_stream
+
+          # Should not raise because `StandardError` is rescued.
+          delete_request = create_rack_request(
+            "DELETE",
+            "/",
+            { "HTTP_MCP_SESSION_ID" => session_id },
+          )
+          response = @transport.handle_request(delete_request)
+          assert_equal 200, response[0]
+          assert_empty @transport.instance_variable_get(:@sessions)
+        end
+
         test "sends notification to correct session with multiple active sessions" do
           # Create first session
           init_request1 = create_rack_request(