OTA: answer once the announced image size has arrived

The Arduino network client treats a half-closed connection as closed,
so the device's reply after the sender's EOF was lost. The header
already carries the image size: UpdateParser::complete() lets the
device finish and answer while the connection is open. ota_push.py
half-closes only if no answer comes within 3 s, for older firmware.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EhqxQ49eCju4CzKYNjZzwT

lib/ota/src/update_parser.h

@@ -50,6 +50,8 @@ class UpdateParser {
 
     void feed(const uint8_t* data, size_t len);
     bool end();  // no more data: true if the update was installed
+    // The whole image the header announced has arrived (the sender need not close the connection).
+    bool complete() const { return state_ == State::Image && received_ == imageSize_; }
 
     State state() const { return state_; }
     const std::string& error() const { return error_; }