_supports_media_in_tool_results() had a hardcoded provider allowlist
that missed custom providers and newer vision-capable providers like
xiaomi. Added ProviderProfile.supports_vision flag and made the
function check:
1. Registered provider profile (supports_vision flag)
2. Model capabilities from models.dev catalog (supports_vision)
3. Existing hardcoded allowlist (unchanged)
This fixes HTTP 400 "text is not set" errors when vision-capable
custom providers receive text-only tool results instead of
multipart image content.
Related: #25594
f736d2be86