ws: make pong sending lazy

[icing]

Do not send PONG frames unless there is sufficient space left in the websocket send buffer. A server might be lazy in reading our data and intermediary PONG frames can be skipped by a client (RFC 6455, ch. 5.5.3).

Add test case measuring no real RSS increase on a server blasting with PING frames.

[Copilot]

Pull request overview

This PR adjusts curl’s WebSocket implementation to avoid sending (and buffering) PONG frames when the WebSocket send buffer is already “too full”, based on RFC 6455’s allowance for skipping intermediary PONGs. It also adds an HTTP-test that stresses the client with a server blasting many PING frames and asserts that the client’s RSS does not materially increase across runs.

Changes:

• Make control-frame (PONG) sending “lazy” by deferring enqueue/flush behavior until there is buffer space.
• Adjust WebSocket decode writer flow to flush pending control frames opportunistically.
• Add a new stress test that measures RSS under heavy PING load.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
tests/http/test_20_websockets.py	Adds a new “crazy pings” stress test that uses profiling RSS to detect memory growth.
lib/ws.c	Defers control-frame sending and adds logic in ws_flush() to only enqueue pending control frames when there is space.

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