fix(openai): improve modality error messages and docs

- Update error messages for unsupported image/PDF inputs with clearer guidance
- Add `modalities` setting to override auto-detected input modalities
- Document `modalities` config in model-providers.md and settings.md
- Update converter tests to match new error message format

This provides users with actionable alternatives when their selected model doesn't support certain input types, and allows manual modality overrides for models not recognized by auto-detection.

Co-authored-by: Qwen-Coder <qwen-coder@alibabacloud.com>

docs/users/configuration/model-providers.md

@@ -64,6 +64,9 @@ This auth type supports not only OpenAI's official API but also any OpenAI-compa
           "maxRetries": 3,
           "enableCacheControl": true,
           "contextWindowSize": 128000,
+          "modalities": {
+            "image": true
+          },
           "customHeaders": {
             "X-Client-Request-ID": "req-123"
           },