fix(vision): detect vision-capable custom providers via ProviderProfile flag

_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

plugins/model-providers/xiaomi/__init__.py

@@ -9,6 +9,7 @@ xiaomi = ProviderProfile(
     env_vars=("XIAOMI_API_KEY",),
     base_url="https://api.xiaomimimo.com/v1",
     supports_health_check=False,  # /v1/models returns 401 even with valid key
+    supports_vision=True,  # mimo-v2-omni is vision-capable
 )
 
 register_provider(xiaomi)