Allow selecting individual WebSocket messages for compression

[mkurz]

This PR lets applications decide whether each outbound WebSocket text or binary message should be compressed.

WebSocket.Accepted now accepts a shouldCompress selector in both the Scala and Java APIs. The selector receives a CompressionContext containing:

• The final outbound Play message
• Its uncompressed payload length in bytes
• Whether it exceeds play.server.websocket.compression.threshold

Returning true or false overrides the configured threshold for that message.

WebSocket.Accepted(
  flow,
  shouldCompress = context =>
    context.message match {
      case TextMessage(text) if text.contains("secret") => false
      case _ => context.isAboveCompressionThreshold
    }
)

This allows applications to avoid compressing sensitive or already-compressed content while continuing to compress other messages on the same WebSocket connection.

Behavior

The selector:

• Is evaluated once for each outbound text or binary message
• Is called only when permessage-deflate was negotiated
• Can override the configured compression threshold
• Cannot enable compression when it is disabled globally or for the accepted WebSocket
• Must not block; an exception fails the WebSocket stream

The default selector preserves the existing threshold behavior.

Implementation

The selector is propagated through message-flow transformations and the Java-to-Scala API bridge.

Both server backends support it:

• Netty uses its WebSocket extension filter for the final outbound frame.
• Pekko HTTP passes the decision to its per-message compression API.

Tests

Shared integration tests for both backends verify:

• Selecting compressed and uncompressed messages on one connection
• Overriding the threshold in both directions
• Text and binary messages
• UTF-8 byte-length calculation
• Control frames bypassing the selector
• The selector not being evaluated when compression is disabled or not negotiated
• Java API propagation
• Actual compressed wire payloads, not only handshake headers

Scala and Java WebSocket documentation and the Play release highlights are updated.

References

• Play per-WebSocket compression control: Allow disabling WebSocket compression per accepted socket #14118
• Play compression threshold: Add a WebSocket compression threshold #14120
• Pekko HTTP per-message compression API: Add selective WebSocket message compression apache/pekko-http#1172
• RFC 7692