feat(FN-3621): harden thinking provider compatibility

- Normalize Anthropic/OpenAI thinking level handling before creating Fn agents
- Add regression tests for thinking-level conflict and provider compatibility behavior
- Document thinking provider compatibility guidance in settings reference
- Ref: Runfusion/Fusion#52

Fusion-Task-Id: FN-3621

docs/settings-reference.md

@@ -38,7 +38,7 @@ Defaults from `DEFAULT_GLOBAL_SETTINGS`; key scope from `GLOBAL_SETTINGS_KEYS`.
 | `defaultModelId` | `string` | `undefined` | Default AI model ID. |
 | `fallbackProvider` | `string` | `undefined` | Fallback provider when the primary default model hits transient provider failures. |
 | `fallbackModelId` | `string` | `undefined` | Fallback model ID (must pair with `fallbackProvider`). |
-| `defaultThinkingLevel` | `"off" \| "minimal" \| "low" \| "medium" \| "high"` | `undefined` | Default reasoning effort for AI sessions. |
+| `defaultThinkingLevel` | `"off" \| "minimal" \| "low" \| "medium" \| "high"` | `undefined` | Default reasoning effort for AI sessions. If a provider/runtime rejects simultaneous `thinking` and `reasoning_effort` parameters, Fusion retries without the explicit thinking override instead of failing the run. |
 | `ntfyEnabled` | `boolean` | `false` | Enable ntfy push notifications. |
 | `ntfyTopic` | `string` | `undefined` | ntfy topic name. |
 | `ntfyBaseUrl` | `string` | `undefined` | Optional custom ntfy server base URL (must use `http://` or `https://`). If blank/unset, Fusion uses `https://ntfy.sh` for both runtime and test notifications. |