ci(wayland-e2e): poll for a warm screenshot instead of a single fixed sleep

[latekvo]

Problem

The Boot + drive AVD on headless Weston job's "Screenshot returns real pixels" step takes a single screenshot 15s after boot_completed and asserts it is > 20 KB. That single shot races two independent warm-up delays:

1. SurfaceFlinger painting the lockscreen UI, and
2. the simulator-server's screenshot stream, which only starts warming on the first capture request.

On slow/contended runners the one-and-only capture comes back as either:

• {"error":"...no image to export"} — a cold stream, response has no .data envelope, so the step's json.load(...)['data'] raises KeyError: 'data'; or
• an all-zero framebuffer that PNG-compresses to ~3–7 KB and fails the > 20 KB check.

Either way the step fails even though the boot was healthy. This is intermittent — the same commit passes most runs and fails some.

Fix

Replace the single fixed sleep 15 + one capture with a bounded poll (15 attempts, 4s apart). Each request also warms the stream, and the loop exits as soon as a frame exceeds 20 KB. A cold-stream error response is tolerated and retried rather than aborting the step. Genuinely broken framebuffers still fail, now with per-attempt diagnostics in the log.

• Healthy boots pass in the first attempt or two (no slower than before in the common case).
• Worst-case added headroom is ~60s, only spent when the framebuffer never warms — which is a real failure worth surfacing.

Verification

The workflow file is unchanged elsewhere; only this step's run: script changed.

• prettier --check passes on the workflow file.
• Extracted the exact run: script and ran it against a mock curl covering: cold-error → all-zero frame → real frame (exits 0 on the real frame); always-cold (exits 1 after 15 attempts); always all-zero framebuffer (exits 1). Exit codes confirmed 0 / 1 respectively.