mcp: improve http transports error handling and make transport work with any size message (#734)

We have a use case for messages larger than 1MB, hence making this
configurable would help.
In order to discover the source of our problems we had to spend some
time investigating, as errors were silently discarded. So we also
included changes to improving surfacing errors .

---------

Co-authored-by: Maciej Kisiel <macio92@gmail.com>

mcp/streamable.go

@@ -1915,6 +1915,14 @@ func (c *streamableClientConn) processStream(ctx context.Context, requestSummary
 			if ctx.Err() != nil {
 				return "", 0, true // don't reconnect: client cancelled
 			}
+
+			// Malformed events are hard errors that indicate corrupted data or protocol
+			// violations. These should fail the connection permanently.
+			if errors.Is(err, errMalformedEvent) {
+				c.fail(fmt.Errorf("%s: %v", requestSummary, err))
+				return "", 0, true
+			}
+
 			break
 		}