fix(encoding): flush codec after each chunk for streaming responses

[Rachit2323]

Fixes #3410

Problem

When using middleware::Compress with a streaming response, chunks don't
arrive at the client progressively — instead everything is buffered and
delivered all at once when the stream ends.

For example, a response that sends one chunk immediately and another after
5 seconds will show nothing for 5 seconds, then both chunks appear at the
same time.

Why it happens

The gzip/deflate/zstd codecs buffer incoming data internally waiting for
more bytes to compress efficiently. write() feeds data in but the codec
decides when to emit output. Without an explicit flush, it holds everything
until finish() is called at stream EOF.

Fix

Added ContentEncoder::flush() which calls sync flush on the underlying
codec, and call it after every write() in poll_next. This forces the
codec to release each chunk immediately rather than waiting for the stream
to end.

Small trade-off: slightly lower compression ratio per chunk, but this is
the same behaviour as nginx/Apache and the right call for streaming.

[JohnTitor]

Please do not paste AI output directly, we don't welcome slops.