fix(xai): accept Grok Build code during loopback wait + tiny screenshot guard

xAI's consent page renders the authorization code in-page instead of
redirecting to the loopback callback, so the listener just hangs and the
manual-paste flow demands a callback URL that never contains the token.

- auth.py: poll stdin non-blockingly while waiting for the xAI loopback
  callback; accept a pasted bare Grok Build code and substitute the locally
  generated state (PKCE code_verifier still binds the exchange). No need to
  wait for timeout or re-run with --manual-paste.
- computer_use: parse PNG/JPEG dimensions from base64 and fall back to the
  text/AX/SOM payload when the screenshot is below the provider minimum
  (8x8), which xAI rejects with HTTP 400.
- model_setup_flows.py: xAI credential reuse prompt uses the standard radio
  picker via a shared _prompt_auth_credentials_choice helper.
- main.py: thread a title through _prompt_provider_choice; re-home the helper
  import (flows live in model_setup_flows.py post-decomposition).

Salvaged from #36781 onto current main (contributor's main.py edits re-homed
to model_setup_flows.py, where the flows were extracted since the PR opened).

tests/tools/test_computer_use_capture_routing.py

@@ -33,10 +33,10 @@ import pytest
 # Fixtures / helpers
 # ---------------------------------------------------------------------------
 
-# 1×1 PNG (transparent) — minimal bytes that decode cleanly.
+# 8×8 PNG (transparent) — minimal provider-acceptable bytes that decode cleanly.
 _PNG_B64 = (
-    "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAQAAAC1HAwCAAAAC0lEQVR42m"
-    "NkYAAAAAYAAjCB0C8AAAAASUVORK5CYII="
+    "iVBORw0KGgoAAAANSUhEUgAAAAgAAAAICAYAAADED76LAAAADUlEQVR4nG"
+    "NgGAUgAAABCAABgukLHQAAAABJRU5ErkJggg=="
 )
 
 # 1×1 JPEG — used to verify mime detection works for either stream type.