Use rescue StandardError for stream close

## Motivation and Context

Replace bare `rescue` with explicit `rescue StandardError` in stream cleanup to clarify intent,
with a comment explaining why.
In Ruby, `rescue` without an exception class is equivalent to `rescue StandardError`,
so this is a readability improvement with no behavior change.

## How Has This Been Tested?

Existing tests pass.

## Breaking Changes

None.

lib/mcp/server/transports/streamable_http_transport.rb

@@ -150,8 +150,8 @@ module MCP
             # 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 @@ module MCP
 
           begin
             session[:stream]&.close
-          rescue
-            nil
+          rescue StandardError
+            # Ignore close-related errors from already closed/broken streams.
           end
           @sessions.delete(session_id)
         end