[Feature] Add content negotiation for /stat endpoint and zstd compression

[Feature] Add content negotiation for /stat endpoint and zstd compression

- Update /stat handler to use rspamd_controller_send_ucl_negotiated
  for Accept header content-type negotiation (JSON/msgpack)
- Add zstd compression support to rspamd_controller_maybe_compress,
  preferred over gzip when client supports it
- Add functional robot tests for content negotiation covering:
  - OpenMetrics/text/plain Accept headers for /metrics
  - JSON/msgpack Accept headers for /stat
  - gzip/zstd Accept-Encoding compression
  - Quality factor parsing