Allow reading multiple data chunks per frame in a single-threaded HTTPRequest

[NoNormalDev]

Problem(s) solved by this PR

• Closes HTTPRequest Non-Threaded Download Speeds are Limited by Framerate #120425

Additional information

When use_threads = false on a HTTPRequest, incoming data gets processed on the main thread. Right now, only one chunk gets read per frame, which means throughput is effectively capped at download_chunk_size * FPS, which can be problematic (see the linked issue for more details)

This PR lets multiple chunks be processed within a single frame.

I also added a read_time_limit property that limits how much time can be spent reading data per frame (this only matters when use_threads = false, it's not used otherwise). Without a limit like this, a fast connection could end up stalling the main for seconds even. At least one read still always happens per frame regardless of the limit.

Here is some data from a project running at 20 FPS, downloading a 16 MiB file on a simulated 10 MB/s network (localhost with a python server):

download_chunk_size	4 KB	65 KB	512 KB
4.7.stable	205 s	13 s	1.9 s
This PR	1.85 s	1.85 s	1.85 s

Author disclosure

I used Claude to discuss possible approaches, though honestly most of its suggestions were awful, and to review the changes for mistakes before pushing.
All the code is mine.