fix(webrtc): harden /sdp HTTP server against memory-amplification DoS

[yashksaini-coder]

Summary

Closes #1354. Recreates #1380, which was auto-closed when the source fork was recreated — same diff, no code changes.

The POST /sdp HTTP handler in _aiortc_helpers.py (run_signaling_server) had two unbounded read paths that let a remote attacker consume server memory or hold async tasks indefinitely:

• Unbounded header loop — while True: with only a per-readline 15s timeout. A slow attacker could send one header line every 14s and hold the connection forever.
• Unbounded body allocation — reader.readexactly(content_length) where content_length is attacker-controlled. Content-Length: 2147483647 grows a ~2 GiB asyncio buffer per connection before the timeout fires.

Fix

Three constants + early rejection before any body buffer is allocated:

Constant	Value	Behaviour on violation
_MAX_SDP_BODY_SIZE	32 KiB	413 Payload Too Large
_MAX_HEADER_LINES	64	400 Bad Request
_MAX_HEADER_BYTES	8 KiB	400 Bad Request

Non-integer and negative Content-Length values also return 400. The existing _SDP_HTTP_TIMEOUT (15s per readline/readexactly) is unchanged. Uses Python's for/else idiom: the else fires when the header loop exhausts _MAX_HEADER_LINES reads without finding the blank-line terminator.

Test plan

• Content-Length: 999999999 → 413, body buffer not allocated
• Content-Length: notanumber → 400
• Content-Length: -1 → 400
• Content-Length exactly at limit → 200 (happy path still works)
• _MAX_HEADER_LINES + 1 non-blank header lines → 400
• Single header line > _MAX_HEADER_BYTES → 400
• Valid SDP request → 200, on_offer callback receives correct body
• All WebRTC tests pass (new test_aiortc_helpers.py regression suite)

Scope note

This harness is documented as a temporary py-to-py scaffold (not interop-compatible with go/js libp2p). The fix raises the bar from "trivially DoSable over LAN" to "needs a real attacker model" while the STUN-based listener (#1352) is in spike. Refs #546, #1309, #1352.